Made changes to some errors prone lines

[shyam314159]

1. Added code in train.py so one can load the model if it already exists and continue training.
2. Added .keras file extension so it shows no error while saving the model or loading the model.

[kralka]

And sorry the reply took me so long, we were quite busy last week.

[coveralls]

Pull Request Test Coverage Report for Build 10630507131

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 80.4%

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Totals
Change from base Build 10661092540:	0.0%
Covered Lines:	2252
Relevant Lines:	2801

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💛 - Coveralls

[kralka]

@shyam314159 do you plan to follow up on this PR?

• I have a todo for the SCA tutorial with GPAM and https://github.com/google/sedpack/pull/25
• I would vote for keeping backwards compatibility and keeping this code as it was (possibly with a note about the older version of TensorFlow).