Imported from GitHub PR #20884

Add int32/string k/v support for tf.contrib.lookup.HashTable
This fix tries to address the issue raised in #20869 where there were no int32/string k/v support for tf.contrib.lookup.HashTable. This fix adds the int32/string for the kernel.

This fix fixes #20869.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

Copybara import of the project:

  - 2cdbb1e05e349b985ace42abd9a6a5140fa38b9f Add int32/string k/v support for tf.contrib.lookup.HashTa... by Yong Tang <yong.tang.github@outlook.com>
  - 54653810d18ea28c4d9fb82736b3529e84d1128c Add test case for int32/string(k/v) of tf.contrib.lookup.... by Yong Tang <yong.tang.github@outlook.com>
  - 27599c2915cde87a3c550c8876519e95b3f828db Fix string vs byte mismatch in python 3 by Yong Tang <yong.tang.github@outlook.com>
  - b58849aac511dcc0fc95218e011e945d3dac86b5 Merge 27599c2915cde87a3c550c8876519e95b3f828db into e7278... by Yong Tang <yong.tang.github@outlook.com>

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/tensorflow/pull/20884 from yongtang:20869-tf.contrib.lookup.HashTable 27599c2915cde87a3c550c8876519e95b3f828db
PiperOrigin-RevId: 205242516