Upgrade to TF 2.7.0.

[MichaelBroughton]

Supersedes #638

Also included:

1. Reverted pip install instructions to correct release version of TensorFlow.
2. Added python snippet to top of all test files to remove conflicting proto definitions on python path. I feel like there should be some way to do this in Bazel, but after trying for a few hours I'm out of ideas.

[zaqqwerty]

Nice, looking forward to using TF 2.7 features with TFQ! Past TF version upgrades sometimes caused performance issues (see #568), how are things looking on that front?

[MichaelBroughton]

Not sure how performance looks. I think a good way to test will be to compare side-by-side some of the tutorial notebooks and see how much slower/faster the training loops get.