Disable CPU Offload as default for DeepSpeed

[SeanNaren]

What does this PR do?

I've had a few reports that the default for DeepSpeed is incredibly slow and is unusable. After investigation, this is because CPU offload by default is set to True, and this can be prohibitive when running DeepSpeed. Instead we should turn off CPU offload, and keep ZeRO on allowing the user to use CPU Offload if necessary.

This can also been seen from the benchmarks here: https://github.com/SeanNaren/minGPT#ddp-vs-deepspeed

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[codecov]

Codecov Report

Merging #6262 (66522bb) into master (3371d32) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #6262   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         159     159           
  Lines       11365   11365           
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- Hits        10608   10579   -29     
- Misses        757     786   +29