[Spec Decode] Disable Log Prob serialization to CPU for spec decoding for both draft and target models.

[sroy745]

In this PR we disable the serialization of the LogProbs to CPU for both draft and target models. To that end we make the following changes

1. Add top level flags to allow users to configure if they want token log probabilities during speculative decoding
2. Introduce a light weight TargetModelRunner using which we can set the SamplingMetadata.skip_sampler_cpu_output as needed.
3. Made changes to SpecDecodeWorker to skip serialization of log probability tensors if not needed.
4. Fixed clearing out the appropriate tensor in SamplerOutput. E.g. earlier we were clearing sampler_output.probs which is not a valid tensor.
5. Fixed failing tests as needed.

[github-actions]

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[sroy745]

@cadedaniel this pr is now ready for review. PTAL.

[cadedaniel]

great stuff, thanks! all feedback is code cleanliness/comments

[cadedaniel]

nit: add comment explaining why we do this

In speculative decoding, the logprobs selected may not be the same ones as
selected by the target model sampling. This means that the time spent in the
logprob calculation of the target model is time wasted, since we calculate
logprobs after deciding which tokens are accepted. For this reason we disable
logprobs in the target model so scoring is faster.

[sroy745]

Done.

[sroy745]

Thanks for the review. Addressed all comments. PTAL.

[cadedaniel]

Enabling auto merge