shard doc improvements (#4993)
shard doc improvements (#4993)

* Rewording

* Update fairscale install link to include bucket fix, add benchmark results

* Added percentage gain

* Address codereview

* Update docs/source/multi_gpu.rst

Co-authored-by: Jirka Borovec <b903e28b3fb12ead55c25f3a7816376be3bd063c@users.noreply.github.com>

* Update multi_gpu.rst

Co-authored-by: Jirka Borovec <b903e28b3fb12ead55c25f3a7816376be3bd063c@users.noreply.github.com>
williamFalcon/pytorch-lightning,williamFalcon/pytorch-lightning