[Bugfix][DCP] Set default CUDAGraphMode to PIECEWISE for DCP

[FENP]

Purpose

#25444 change default CUDAGraphMode from PIECEWISE to FULL_AND_PIECEWISE. However, DCP do not support full cuda graphs now (#26022 (comment)). This PR change default CUDAGraphMode to PIECEWISE when enable DCP.

cc @youzhedian @youkaichao @LucasWilkinson

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request addresses an issue where enabling Decode Context Parallelism (DCP) was incompatible with full CUDA graph modes. The proposed fix forces the cudagraph_mode to PIECEWISE when DCP is active. While the intent is correct, the implementation is overly aggressive and will override a user's explicit choice to disable CUDA graphs entirely (cudagraph_mode=NONE). My review includes a critical comment to refine this logic, ensuring it only downgrades from FULL modes to PIECEWISE and warns the user, without affecting NONE mode.

[gemini-code-assist]

This implementation unconditionally sets cudagraph_mode to PIECEWISE if decode context parallelism (DCP) is enabled. This is too aggressive as it will override a user's explicit choice to disable CUDA graphs (e.g., cudagraph_mode=NONE), which might be done for debugging purposes.

A better approach is to only downgrade the mode to PIECEWISE if a FULL CUDA graph mode was requested, as those are the ones incompatible with DCP. This change also adds a warning to inform the user about the automatic adjustment.

if self.parallel_config.decode_context_parallel_size > 1 and \
                    self.compilation_config.cudagraph_mode.has_full_cudagraphs():
                    logger.warning(
                        "Decode context parallel (DCP) is enabled, which is "
                        "incompatible with full CUDA graphs. Downgrading "
                        "cudagraph_mode from %s to PIECEWISE.",
                        self.compilation_config.cudagraph_mode.name)
                    self.compilation_config.cudagraph_mode = CUDAGraphMode.PIECEWISE

[FENP]

These code snippets will only execute when cudagraph_mode is not explicitly set by users.

[youkaichao]

cc @LucasWilkinson @youzhedian it should be possible to make dcp compatible with full cudagraph.