Allow optimizer CG to share the same pool as full-iter CG

[nanz-nv]

What does this PR do ?

Allow optimizer cuda graph and eval graph to share the same memory pool as the full-iter train cuda graph. This can be done through cuda_graph_use_single_mempool, which is default to True. This flag was used by local partial cuda graph to opt in for mempool sharing among graphs. Updated the doc string for this knob as the local cuda graph now doesn't rely on this flag anymore and does the sharing unconditionally.

dev PR: #4521

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