Fix two issues related to `--moe-dense-tp-size=1`

[ch-wan]

Motivation

There were two issues related to --moe-dense-tp-size=1 with DP attention:

• The CUDA graph runner overestimated the buffer size required by DP attention, which may prevent the execution of cuda graph (#5527)
• dp<tp is not executable when --moe-dense-tp-size=1 (#5656).

This PR fixes the two issues. Note that fixing the cuda graph issue requries #5558.

Checklist

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