Update default max_num_batch_tokens for chunked prefill to 2048

[mgoin]

We have seen that 512 is a very conservative value, especially once on H100. Increasing this is 2048 is still a bit conservative but a definite improvement in processing large prefills or throughput.

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @mgoin.

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