Keep global step update in the loop

[carmocca]

What does this PR do?

The gradient accumulation logic check and global step update should be contained within the loops.
This PR changes the TTP API so that we only use it to know whether the optimization is handled internally in the case of the IPU and DeepSpeed plugins.

Does your PR introduce any breaking changes? If yes, please list them.

Beta TrainingTypePlugin and Accelerator API:

• Replaced update_global_step with the accumulate_grad_batches property.

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[codecov]

Codecov Report

Merging #8856 (6c509f2) into master (b294c57) will decrease coverage by 4%.
The diff coverage is 57%.

@@           Coverage Diff           @@
##           master   #8856    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         180     180            
  Lines       15062   15051    -11     
=======================================
- Hits        13983   13369   -614     
- Misses       1079    1682   +603     

[tchaton]

LGTM ! Nice cleanup.