[Model] Clean up and simplify Mamba2 Metadata Usage in both V0 and V1

[cyang49]

Purpose

This PR cleans up some naming inconsistencies in mamba2 metadata between V0 and V1. This affects models that use mamba_mixer2 or plamo2

Test Plan

Run e2e lm_eval on ibm-granite/granite-4.0-tiny-preview and pfnet/plamo-2.1-2b-cpt for both V0 and V1

Test Result

• No runtime errors
• Accuracy is maintained
  They passed and no significant accuracy differences observed.

granite-4-tiny-preview V1 (main 006e7a3)

VLLM_USE_V1=1 lm_eval --model vllm  --model_args pretrained=ibm-granite/granite-4.0-tiny-preview,tensor_parallel_size=1,dtype=auto,gpu_memory_utilization=0.95,enable_prefix_caching=False,max_model_len=8192 --batch_size auto --trust_remote_code  --cache_requests true --tasks gsm8k

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.6035|±  |0.0135|
|     |       |strict-match    |     5|exact_match|↑  |0.5754|±  |0.0136|

granite-4-tiny-preview V1 (this PR)

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.6088|±  |0.0134|
|     |       |strict-match    |     5|exact_match|↑  |0.5861|±  |0.0136|

granite-4-tiny-preview V0 (this PR)

VLLM_USE_V1=0 lm_eval --model vllm  --model_args pretrained=ibm-granite/granite-4.0-tiny-preview,tensor_parallel_size=1,dtype=auto,gpu_memory_utilization=0.95,enable_prefix_caching=False,max_model_len=8192 --batch_size auto --trust_remote_code  --cache_requests true --tasks gsm8k

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value|   |Stderr|
|-----|------:|----------------|-----:|-----------|---|----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.602|±  |0.0135|
|     |       |strict-match    |     5|exact_match|↑  |0.583|±  |0.0136|

Plamo2 V1 (main 006e7a3)

VLLM_USE_V1=1 lm_eval --model vllm  --model_args pretrained=pfnet/plamo-2.1-2b-cpt,tensor_parallel_size=1,dtype=auto,gpu_memory_utilization=0.95,enable_prefix_caching=False,max_model_len=8192 --batch_size auto --trust_remote_code  --cache_requests true --tasks gsm8k

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.5898|±  |0.0135|
|     |       |strict-match    |     5|exact_match|↑  |0.5823|±  |0.0136|

Plamo2 V1 (this PR)

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.5898|±  |0.0135|
|     |       |strict-match    |     5|exact_match|↑  |0.5823|±  |0.0136|

Plamo2 V0 (this PR)

VLLM_USE_V1=0 lm_eval --model vllm  --model_args pretrained=pfnet/plamo-2.1-2b-cpt,tensor_parallel_size=1,dtype=auto,gpu_memory_utilization=0.95,enable_prefix_caching=False,max_model_len=8192 --batch_size auto --trust_remote_code  --cache_requests true --tasks gsm8k

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.5982|±  |0.0135|
|     |       |strict-match    |     5|exact_match|↑  |0.5951|±  |0.0135|

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request refactors the Mamba2 metadata handling to unify the logic for V0 and V1 APIs. The changes primarily involve renaming metadata fields to be consistent across both versions, which simplifies the code in mamba_mixer2 and plamo2 by removing duplicated logic. The change to how has_initial_states_p is calculated is also a good improvement for correctness and clarity. I've found one critical issue where attribute assignments were not updated after renaming, which could lead to a runtime error.

[tdoublep]

LGTM