New speed documentation

[edenlightning]

1/N docs restructure #7666.

Includes a guide to speed up model training:

• GPU/TPU training
• Mixed precision (16-bit) training
• Control Training Epochs
• Control Validation Frequency
• Limit Dataset Size
• Preload Data Into RAM
• Model Toggling
• Set Grads to None
• Things to avoid

[codecov]

Codecov Report

Merging #7665 (7cf96ea) into master (55494e8) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #7665   +/-   ##
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- Coverage      92%     91%   -0%     
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  Files         207     207           
  Lines       13374   13374           
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- Hits        12246   12208   -38     
- Misses       1128    1166   +38     

[kaushikb11]

Could we rename it to performance instead?

[edenlightning]

Thanks for the review @kaushikb11 ! You mean rename the guide? What would you call it? The idea is to make these guides actionable... like how to do X

[awaelchli]

I really love all the "use when" additions! Great idea <3

[ethanwharris]

LGTM, minor suggestions 😃

[justusschock]

One general question: Similar to those .. note:: or .. warning:: boxes, do you think it makes sense to add boxes (maybe in lightning-lila) for .. use-when:: as well? So that the use cases are even more highlighted? (Not necessarily for this PR, but in general/long-term)

[edenlightning]

@justusschock great suggestion!! Any idea how to do that?

[justusschock]

@edenlightning From what I just read, this should be possible with a combination of this, this and this. Unfortunately I really don't know myself around with CSS and that stuff at all, so I really cannot take this one, sorry!

[aribornstein]

LGTM

[aribornstein]

LGTM