[TRTLLM-13077][feat] Deocmpose post_load_weights()

[chienchunhung]

@coderabbitai summary

Summary

Add the prep layer for staged post-load hooks without migrating existing model implementations yet.

• Add default setup_aliases(), transform_weights(), and cache_derived_state() hooks on DecoderModelForCausalLM, with post_load_weights() preserving the current full sequence.
• Add duck-typed ModelLoader helpers for top-level alias setup, transform walks, cache-state walks, full post-load walks, and _weights_transformed reset.
• Keep reload() behavior unchanged; reload remains weight-copy-only and does not run post-load hooks.
• Add focused mock tests for staged-hook helper behavior.

Test Plan

• python3 -m pytest tests/unittest/_torch/pyexecutor/test_model_loader_mx.py -q

Note: local run was blocked in this environment by missing built tensorrt_llm.bindings; should be run in the normal TRT-LLM build/test environment.

PR Checklist

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[chienchunhung]

/bot run --disable-fail-fast

[tensorrt-cicd]

PR_Github #51131 [ run ] triggered by Bot. Commit: d93df17 Link to invocation

[tensorrt-cicd]

PR_Github #51131 [ run ] completed with state SUCCESS. Commit: d93df17
/LLM/main/L0_MergeRequest_PR pipeline #40568 completed with status: 'FAILURE'

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