Get rid of any pyflakes/PEP8 warning in tensorflow module

[Muammar El Khatib]

There are two tools letting a developer check that a python module is easy to read and follow the recommended standard (PEP) and that there are no errors (pyflakes). The objective is to get rid of all of the possible warnings. This same task applies to all .py files in Amp.

[andrew_peterson]

Cleanup tflow a bit.

Addresses issue #196.

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