[TRTLLM-11508][refactor] decouple MTP num_nextn_predict_layers from max_draft_len

[zhaoyangwang-nvidia]

…ax_draft_len

@coderabbitai summary

Description

The internal field num_nextn_predict_layers_from_model_config has been removed and replaced by num_nextn_predict_layers.

The original num_nextn_predict_layers field in MTPDecodingConfig, which conflated two separate concerns, was split into two fields with clear responsibilities:

Field	Source	Role
max_draft_len	User-facing	Controls how many draft tokens to produce
num_nextn_predict_layers	Auto-populated from model (internal)	How many MTP layers actually exist in the checkpoint

Parameter Logic Per Mode

Eagle MTP (e.g. DeepSeek-V3, model has only 1 MTP layer)

• num_nextn_predict_layers = 1 (read from model)
• max_draft_len = N (set by user, default 1)
• Behavior: runs the single MTP layer N times, producing N draft tokens

Vanilla MTP (model has multiple MTP layers)

• num_nextn_predict_layers = M (read from model)
• User does not set max_draft_len → automatically uses M, runs all layers
• User explicitly sets max_draft_len = N:
	○ N < M: prints a warning, uses N layers, produces N draft tokens

N >= M: uses M, produces M draft tokens

Test Coverage

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