[RLlib] Deprecate AlgorithmConfig.framework("tfe"): Use tf2 instead.

[sven1977]

Signed-off-by: sven1977 svenmika1977@gmail.com

Deprecate AlgorithmConfig.framework("tfe"): Use tf2 instead.

tfe and tf2 have the same meaning. There used to be a difference in the tf main-version (1.x vs 2.x) for these two settings, but that difference has already been equalized in RLlib's code.
We are now throwing an error, hinting to use "tf2", if a user tries to set framework="tfe".

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