Replaced the scalar type for string tensors from std::string to tensorflow::tstring which is now ABI stable.
A new Profiler for TF 2 for CPU/GPU/TPU. It offers both device and host performance analysis, including input pipeline and TF Ops. Optimization advisory is provided whenever possible. Please see this tutorial and guide for usage guidelines.
Export C++ functions to Python using pybind11 as opposed to SWIG as a part of our deprecation of swig efforts.
tf.distribute:
- Support added for global sync BatchNormalization by using the newly added tf.keras.layers.experimental.SyncBatchNormalization layer. This layer will sync BatchNormalization statistics every step across all replicas taking part in sync training.
- Performance improvements for GPU multi-worker distributed training using tf.distribute.experimental.MultiWorkerMirroredStrategy
- Update NVIDIA NCCL to 2.5.7-1 for better performance and performance tuning. Please see nccl developer guide for more information on this.
- Support gradient allreduce in float16. See this example usage.
- Experimental support of all reduce gradient packing to allow overlapping gradient aggregation with backward path computation.
tf.keras:
- Model.fit major improvements:
  - You can now use custom training logic with Model.fit by overriding Model.train_step.
  - Easily write state-of-the-art training loops without worrying about all of the features Model.fit handles for you (distribution strategies, callbacks, data formats, looping logic, etc)
  - See the default Model.train_step for an example of what this function should look like
  - Same applies for validation and inference via Model.test_step and Model.predict_step
- The SavedModel format now supports all Keras built-in layers (including metrics, preprocessing layers, and stateful RNN layers)
tf.lite:
- Enable TFLite experimental new converter by default.
XLA
- XLA now builds and works on windows. All prebuilt packages come with XLA available.
- XLA can be enabled for a tf.function with “compile or throw exception” semantics on CPU and GPU.

Breaking Changes

tf.keras:
- In tf.keras.applications the name of the "top" layer has been standardized to "predictions". This is only a problem if your code relies on the exact name of the layer.
- Huber loss function has been updated to be consistent with other Keras losses. It now computes mean over the last axis of per-sample losses before applying the reduction function.
AutoGraph no longer converts functions passed to tf.py_function, tf.py_func and tf.numpy_function.
Deprecating XLA_CPU and XLA_GPU devices with this release.
Increasing the minimum bazel version to build TF to 2.0.0 to use Bazel's cc_experimental_shared_library.

Known Caveats

MacOS binaries are not available on pypi at tensorflow-cpu project, but they are identical to the binaries in tensorflow project, since MacOS has no GPU.
Due to certain unforeseen circumstances, we are unable to release MacOS py3.8 binaries, but Windows/Linux binaries for py3.8 are available.
The current TensorFlow release now requires gast version 0.3.3.

Bug Fixes and Other Changes

tf.data:
- Removed autotune_algorithm from experimental optimization options.
TF Core:
- tf.constant always creates CPU tensors irrespective of the current device context.
- Eager TensorHandles maintain a list of mirrors for any copies to local or remote devices. This avoids any redundant copies due to op execution.
- For tf.Tensor & tf.Variable, .experimental_ref() is no longer experimental and is available as simply .ref().
- pfor/vectorized_map: Added support for vectorizing 56 more ops. Vectorizing tf.cond is also supported now.
- Set as much partial shape as we can infer statically within the gradient impl of the gather op.
- Gradient of tf.while_loop emits StatelessWhile op if cond and body functions are stateless. This allows multiple gradients while ops to run in parallel under distribution strategy.
- Speed up GradientTape in eager mode by auto-generating list of op inputs/outputs which are unused and hence not cached for gradient functions.
- Support back_prop=False in while_v2 but mark it as deprecated.
- Improve error message when attempting to use None in data-dependent control flow.
- Add RaggedTensor.numpy().
- Update RaggedTensor.__getitem__ to preserve uniform dimensions & allow indexing into uniform dimensions.
- Update tf.expand_dims to always insert the new dimension as a non-ragged dimension.
- Update tf.embedding_lookup to use partition_strategy and max_norm when ids is ragged.
- Allow batch_dims==rank(indices) in tf.gather.
- Add support for bfloat16 in tf.print.
tf.distribute:
- Support embedding_column with variable-length input features for MultiWorkerMirroredStrategy.
tf.keras:
- Added all_reduce_sum_gradients argument to tf.keras.optimizer.Optimizer.apply_gradients. This allows custom gradient aggregation and processing aggregated gradients in custom training loop.
- Allow pathlib.Path paths for loading models via Keras API.
tf.function/AutoGraph:
- AutoGraph is now available in ReplicaContext.merge_call, Strategy.extended.update and Strategy.extended.update_non_slot.
- Experimental support for shape invariants has been enabled in tf.function. See the API docs for tf.autograph.experimental.set_loop_options for additonal info.
- AutoGraph error messages now exclude frames corresponding to APIs internal to AutoGraph.
- Improve shape inference for tf.function input arguments to unlock more Grappler optimizations in TensorFlow 2.x.
- Improve automatic control dependency management of resources by allowing resource reads to occur in parallel and synchronizing only on writes.
- Fix execution order of multiple stateful calls to experimental_run_v2 in tf.function.
- You can now iterate over RaggedTensors using a for loop inside tf.function.
tf.lite:
- Migrated the tf.lite C inference API out of experimental into lite/c.
- Add an option to disallow NNAPI CPU / partial acceleration on Android 10
- TFLite Android AARs now include the C headers and APIs are required to use TFLite from native code.
- Refactors the delegate and delegate kernel sources to allow usage in the linter.
- Limit delegated ops to actually supported ones if a device name is specified or NNAPI CPU Fallback is disabled.
- TFLite now supports tf.math.reciprocal1 op by lowering to tf.div op.
- TFLite's unpack op now supports boolean tensor inputs.
- Microcontroller and embedded code moved from experimental to main TFLite folder
- Check for large TFLite tensors.
- Fix GPU delegate crash with C++17.
- Add 5D support to TFLite strided_slice.
- Fix error in delegation of DEPTH_TO_SPACE to NNAPI causing op not to be accelerated.
- Fix segmentation fault when running a model with LSTM nodes using NNAPI Delegate
- Fix NNAPI delegate failure when an operand for Maximum/Minimum operation is a scalar.
- Fix NNAPI delegate failure when Axis input for reduce operation is a scalar.
- Expose option to limit the number of partitions that will be delegated to NNAPI.
- If a target accelerator is specified, use its feature level to determine operations to delegate instead of SDK version.
tf.random:
- Various random number generation improvements:
- Add a fast path for default random_uniform
- random_seed documentation improvement.
- RandomBinomial broadcasts and appends the sample shape to the left rather than the right.
- Added tf.random.stateless_binomial, tf.random.stateless_gamma, tf.random.stateless_poisson
- tf.random.stateless_uniform now supports unbounded sampling of int types.
Math and Linear Algebra:
- Add tf.linalg.LinearOperatorTridiag.
- Add LinearOperatorBlockLowerTriangular
- Add broadcasting support to tf.linalg.triangular_solve#26204, tf.math.invert_permutation.
- Add tf.math.sobol_sample op.
- Add tf.math.xlog1py.
- Add tf.math.special.{dawsn,expi,fresnel_cos,fresnel_sin,spence}.
- Add a Modified Discrete Cosine Transform (MDCT) and its inverse to tf.signal.
TPU Enhancements:
- Refactor TpuClusterResolver to move shared logic to a separate pip package.
- Support configuring TPU software version from cloud tpu client.
- Allowed TPU embedding weight decay factor to be multiplied by learning rate.
XLA Support:
- Add standalone XLA AOT runtime target + relevant .cc sources to pip package.
- Add check for memory alignment to MemoryAllocation::MemoryAllocation() on 32-bit ARM. This ensures a deterministic early exit instead of a hard to debug bus error later.
- saved_model_cli aot_compile_cpu allows you to compile saved models to XLA header+object files and include them in your C++ programs.
- Enable Igamma, Igammac for XLA.
- XLA reduction emitter is deterministic when the environment variable TF_DETERMINISTIC_OPS is set.
Tracing and Debugging:
- Add source, destination name to _send traceme to allow easier debugging.
- Add traceme event to fastpathexecute.
Other:
- Fix an issue with AUC.reset_states for multi-label AUC #35852
- Fix the TF upgrade script to not delete files when there is a parsing error and the output mode is in-place.
- Move tensorflow/core:framework/*_pyclif rules to tensorflow/core/framework:*_pyclif.

Thanks to our Contributors

This release contains contributions from many people at Google, as well as:

372046933, 8bitmp3, aaronhma, Abin Shahab, Aditya Patwardhan, Agoniii, Ahti Kitsik, Alan Yee, Albin Joy, Alex Hoffman, Alexander Grund, Alexandre E. Eichenberger, Amit Kumar Jaiswal, amoitra, Andrew Anderson, Angus-Luo, Anthony Barbier, Anton Kachatkou, Anuj Rawat, archis, Arpan-Dhatt, Arvind Sundararajan, Ashutosh Hathidara, autoih, Bairen Yi, Balint Cristian, Bas Aarts, BashirSbaiti, Basit Ayantunde, Ben Barsdell, Benjamin Gaillard, boron, Brett Koonce, Bryan Cutler, Christian Goll, Christian Sachs, Clayne Robison, comet, Daniel Falbel, Daria Zhuravleva, darsh8200, David Truby, Dayananda-V, deepakm, Denis Khalikov, Devansh Singh, Dheeraj R Reddy, Diederik Van Liere, Diego Caballero, Dominic Jack, dothinking, Douman, Drake Gens, Duncan Riach, Ehsan Toosi, ekuznetsov139, Elena Zhelezina, elzino, Ending2015a, Eric Schweitz, Erik Zettel, Ethan Saadia, Eugene Kuznetsov, Evgeniy Zheltonozhskiy, Ewout Ter Hoeven, exfalso, FAIJUL, Fangjun Kuang, Fei Hu, Frank Laub, Frederic Bastien, Fredrik Knutsson, frreiss, Frédéric Rechtenstein, fsx950223, Gaurav Singh, gbaned, George Grzegorz Pawelczak, George Sterpu, Gian Marco Iodice, Giorgio Arena, Hans Gaiser, Hans Pabst, Haoyu Wu, Harry Slatyer, hsahovic, Hugo, Hugo Sjöberg, IrinaM21, jacco, Jake Tae, Jean-Denis Lesage, Jean-Michel Gorius, Jeff Daily, Jens Elofsson, Jerry Shih, jerryyin, Jin Mingjian, Jinjing Zhou, JKIsaacLee, jojimonv, Jonathan Dekhtiar, Jose Ignacio Gomez, Joseph-Rance, Judd, Julian Gross, Kaixi Hou, Kaustubh Maske Patil, Keunwoo Choi, Kevin Hanselman, Khor Chean Wei, Kilaru Yasaswi Sri Chandra Gandhi, Koan-Sin Tan, Koki Ibukuro, Kristian Holsheimer, kurileo, Lakshay Tokas, Lee Netherton, leike666666, Leslie-Fang-Intel, Li, Guizi, LIUJIAN435, Lukas Geiger, Lyo Nguyen, madisetti, Maher Jendoubi, Mahmoud Abuzaina, Manuel Freiberger, Marcel Koester, Marco Jacopo Ferrarotti, Markus Franke, marload, Mbah-Javis, mbhuiyan, Meng Zhang, Michael Liao, MichaelKonobeev, Michal Tarnowski, Milan Straka, minoring, Mohamed Nour Abouelseoud, MoussaMM, Mrinal Jain, mrTsjolder, Måns Nilsson, Namrata Bhave, Nicholas Gao, Niels Ole Salscheider, nikochiko, Niranjan Hasabnis, Nishidha Panpaliya, nmostafa, Noah Trenaman, nuka137, Officium, Owen L - Sfe, Pallavi G, Paul Andrey, Peng Sun, Peng Wu, Phil Pearl, PhilipMay, pingsutw, Pooya Davoodi, PragmaTwice, pshiko, Qwerty71, R Gomathi, Rahul Huilgol, Richard Xiao, Rick Wierenga, Roberto Rosmaninho, ruchit2801, Rushabh Vasani, Sami, Sana Damani, Sarvesh Dubey, Sasan Jafarnejad, Sergii Khomenko, Shane Smiskol, Shaochen Shi, sharkdtu, Shawn Presser, ShengYang1, Shreyash Patodia, Shyam Sundar Dhanabalan, Siju Samuel, Somyajit Chakraborty Sam, Srihari Humbarwadi, srinivasan.narayanamoorthy, Srishti Yadav, Steph-En-M, Stephan Uphoff, Stephen Mugisha, SumanSudhir, Taehun Kim, Tamas Bela Feher, TengLu, Tetragramm, Thierry Herrmann, Tian Jin, tigertang, Tom Carchrae, Tom Forbes, Trent Lo, Victor Peng, vijayphoenix, Vincent Abriou, Vishal Bhola, Vishnuvardhan Janapati, vladbataev, VoVAllen, Wallyss Lima, Wen-Heng (Jack) Chung, wenxizhu, William D. Irons, William Zhang, Xiaoming (Jason) Cui, Xiaoquan Kong, Xinan Jiang, Yasir Modak, Yasuhiro Matsumoto, Yaxun (Sam) Liu, Yong Tang, Ytyt-Yt, yuan, Yuan Mingshuai, Yuan Tang, Yuki Ueda, Yusup, zhangshijin, zhuwenxi