Remove tbptt self.log flags and other dead code [5/n]

[carmocca]

What does this PR do?

Remove support for self.log(tbptt_reduce_fx) and self.log(tbptt_pad_token). The flags have been kept in case somebody is passing default kwargs. GitHub code search or google search did not provide any actual uses of these flags, we also don't have any tests for them or mention in the docs.

Also remove the unused gather and padded_gather functions.

Part of #7183

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[codecov]

Codecov Report

Merging #7644 (ff0e656) into master (e16d4fb) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7644   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         198     198           
  Lines       12806   12783   -23     
======================================
- Hits        11835   11789   -46     
- Misses        971     994   +23     

[tchaton]

LGTM !