Existing `logprobs_from_logits_v2` doesnt achieve the memory savings it
claims. This is because `logsumexp` still allocates a `bs*seqlen*vocab`
tensor internally to hold the element-wise application of `exp`.
However, by applying a loop over `logsumexp`, we can iteratively compute
logsumexp outputs.

Benchmarks show this uses significantly less memory to compute logprobs.

Fix provided, as well as a separate memory-efficient approach for
bfloat16 case.