decouple embedding creation from optimizer

[zhuofan1123]

This PR refactors the embedding creation interface, decoupling it from the optimizer dependency. Users now can designate the embeddings for optimization during optimizer initialization.
cpp:

wholememory_create_embedding(&wm_embedding, ...);
wholememory_create_embedding_optimizer(&optimizer, ...);
wholememory_embedding_set_optimizer(wm_embedding, optimizer);

python:

wm_embedding = wgth.create_embedding(...)
wm_optimizer = wgth.create_wholememory_optimizer(wm_embedding, "adam", {})

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[linhu-nv]

seems good to me.

[BradReesWork]

/okay to test

[BradReesWork]

/merge