[IR2Vec] Restrict caching only to Flow-Aware computation (#162559)

Removed all the caching maps (BB, Inst) in `Embedder` as we don't want
to cache embeddings in general. Our earlier experiments on Symbolic
embeddings show recomputation of embeddings is cheaper than cache
lookups.

OTOH, Flow-Aware embeddings would benefit from instruction level
caching, as computing the embedding for an instruction would depend on
the embeddings of other instructions in a function. So, retained
instruction embedding caching logic only for Flow-Aware computation.
This also necessitates an `invalidate` method that would clean up the
cache when the embeddings would become invalid due to transformations.

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