PR to resolve issue #1140 - AttributeError: module 'tensorflow._api.v2.train' has no attribute 'Optimizer'

[abhilash1910]

This PR has a minor modification to remove the error due to tensorflow version (>=2) incompatibility for optimizer.py.
Due to deprecation of 'tf.train.Optimizer', an improvement is made which allows the optimizer to run smoothly with tf version>=2(tested locally with 2.3.1) .
Suggestions are appreciated.Thanks

[abhilash1910]

Changes:

1. Provided python 2 support for _run_split_on_punc (tokenization.py)
2. Provided tf2 options in modeling.py and modeling_test.py

[chetaspahuja]

[ghost]

Changes:

1. Provided python 2 support for _run_split_on_punc  (tokenization.py)

2. Provided tf2  options in modeling.py and modeling_test.py

@abhilash1910 , for tensorflow 2.16.x, python throws an error message when modeling.BertModel() gets called.

AttributeError: Tensor.name is undefined when eager execution is enabled.

Essentially, the issue is with name = tensor.name on line 911 in modeling.py.