[Feat] Add FastForwardSampler 2/n - Fault Tolerant Training

[tchaton]

What does this PR do?

This PR is part of the Fault Tolerant Training work.

Linked to PoC: #8131

This adds the FastForwardSampler, CaptureIterativeDataset and bunch of tests to insure right behaviour.

Fixes #<issue_number>

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[pep8speaks]

Hello @tchaton! Thanks for updating this PR.

• In the file tests/utilities/test_auto_restart.py:

Line 390:13: W503 line break before binary operator

Comment last updated at 2021-07-07 19:31:12 UTC

[codecov]

Codecov Report

Merging #8307 (6daff95) into master (bca5adf) will decrease coverage by 4%.
The diff coverage is 91%.

@@           Coverage Diff           @@
##           master   #8307    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         213     214     +1     
  Lines       13798   13900   +102     
=======================================
- Hits        12744   12217   -527     
- Misses       1054    1683   +629     

[ethanwharris]

LGTM, small comments

[awaelchli]

accept to unblock <3