Start working on TensorFlow 2.0 compatibility

[gpengzhi]

This PR mainly contains hyperparams.py, module_base.py, utils/mode.py, and utils/utils.py.

embedders module added.
networks module added.
encoders module added.
pretrained module added.
data module added.
tokenizer module added.

[ZhitingHu]

Where is this file docs/_static/img/texar_modules_big.png from?

[gpengzhi]

Where is this file docs/_static/img/texar_modules_big.png from?

Same picture we used in texar-pytorch.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (TensorFlow-2.0@676c66a). Click here to learn what that means.
The diff coverage is 57.89%.

@@                Coverage Diff                @@
##             TensorFlow-2.0     #250   +/-   ##
=================================================
  Coverage                  ?   74.74%           
=================================================
  Files                     ?       69           
  Lines                     ?     4055           
  Branches                  ?        0           
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  Hits                      ?     3031           
  Misses                    ?     1024           
  Partials                  ?        0

Impacted Files	Coverage Δ
texar/tf/data/data/data_iterators.py	82.35% <ø> (ø)
texar/tf/modules/embedders/embedder_utils.py	92% <ø> (ø)
texar/tf/modules/embedders/__init__.py	100% <ø> (ø)
texar/tf/data/data/tfrecord_data_test.py	93.61% <ø> (ø)
texar/tf/hyperparams_test.py	95.94% <ø> (ø)
texar/tf/modules/embedders/embedder_base.py	93.87% <ø> (ø)
texar/tf/data/data/tfrecord_data.py	93.33% <ø> (ø)
texar/tf/data/vocabulary.py	80.72% <ø> (ø)
texar/tf/modules/encoders/transformer_encoder.py	95.74% <ø> (ø)
texar/tf/data/data/data_base.py	86.66% <ø> (ø)
... and 34 more

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[gpengzhi]

Close this PR for now for development, Will reopen it soon.