[Speculative decoding] [Bugfix] Fix overallocation in ngram + spec logprobs #4672
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There's a bug introduced in my spec logprobs PR that only shows up for ngram. We missed it because the ngram tests weren't actually running until recently (#4551).
The bug is that the output logprob tensor of shape (batch_size, num_speculation, vocab_size, sizeof(float32)) is returned once per step, instead of slicing it into (batch_size, 1, vocab_size, sizeof(float32)) views. The spec decode framework does a torch.stack to make contiguous the logprobs of all steps; this caused quadratic memory allocation in num_steps. for a high enough batch size it can cause ooms (e.g. 64).
This is fixed by appropriately slicing the output logprobs.