Add support for (accelerator='cpu'|'gpu'|'tpu'|'ipu'|'auto')

[kaushikb11]

What does this PR do?

Breaking down #6090 into three parts:

• Add support for (accelerator='cpu'|'gpu'|'tpu'|'auto')
• Add devices flag which would be generic for all accelerators
• Add training_type flag mapped to Training Plugins.

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• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

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[codecov]

Codecov Report

Merging #7808 (a2847ec) into master (3102922) will decrease coverage by 5%.
The diff coverage is 81%.

@@           Coverage Diff           @@
##           master   #7808    +/-   ##
=======================================
- Coverage      93%     88%    -5%     
=======================================
  Files         215     215            
  Lines       13988   14034    +46     
=======================================
- Hits        12965   12342   -623     
- Misses       1023    1692   +669     

[pep8speaks]

Hello @kaushikb11! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-07-09 14:59:07 UTC

[awaelchli]

accept to unblock

[ethanwharris]

LGTM, small comments 😃