Merge pull request tensorflow#31898 from mrry/cherrypicks_CPD6K

[r2.0 cherrypick] Fix tf.gradients() performance regression

Use "numpy >= 1.16.0, < 2.0" to fix tensorflow#25636

Update install_python3.6_pip_packages.sh to use apt.

Now python3.6 can be installed from apt and it will be installed with
all submodules.

If we're compiling Python from source, during compilation we get:

```
The necessary bits to build these optional modules were not found:
_bz2                  _dbm                  _gdbm
_lzma                 _sqlite3              _tkinter
readline
```

which then results in

```
==================== Test output for //bazel_pip/tensorflow/contrib/summary:summary_ops_test:
Running test /tmpfs/src/github/tensorflow/bazel-ci_build-cache/.cache/bazel/_bazel_kbuilder/eab0d61a99b6696edb3d2aff87b585e8/execroot/org_tensorflow/bazel-out/k8-opt/bin/bazel_pip/tensorflow/contrib/summary/summary_ops_test.runfiles/org_tensorflow/bazel_pip/tensorflow/contrib/summary/summary_ops_test  on GPU 0
Traceback (most recent call last):
  File "/tmpfs/src/github/tensorflow/bazel-ci_build-cache/.cache/bazel/_bazel_kbuilder/eab0d61a99b6696edb3d2aff87b585e8/execroot/org_tensorflow/bazel-out/k8-opt/bin/bazel_pip/tensorflow/contrib/summary/summary_ops_test.runfiles/org_tensorflow/bazel_pip/tensorflow/contrib/summary/summary_ops_test.py", line 23, in <module>
    import sqlite3
  File "/usr/local/lib/python3.6/sqlite3/__init__.py", line 23, in <module>
    from sqlite3.dbapi2 import *
  File "/usr/local/lib/python3.6/sqlite3/dbapi2.py", line 27, in <module>
    from _sqlite3 import *
ModuleNotFoundError: No module named '_sqlite3'
================================================================================
```

and similar failures which then block releasing patch version.

Add Sergii Khomenko to contributor list

tensorflow@dd551ab
tensorflow@cb93088

Update release notes.

Merge pull request tensorflow#29548 from tensorflow/bananabowl-patch-2

Copy cl/250062993

Merge pull request tensorflow#29521 from goldiegadde/ggadde-cp2

Revert "Fix an important performance regression for LSTM and GRU in t…

Merge pull request tensorflow#28973 from bananabowl/cherrypicks_NL20I

Two cherrypicks

Merge pull request tensorflow#27959 from tensorflow/update-release-no…

…tes-version

Update header on release notes

Merge pull request tensorflow#27699 from yongtang:27497-ragged-axis

PiperOrigin-RevId: 244173513