Feat: Eagle3 HF Online - support nemotron and VLMs (#463)

## What does this PR do?

**Type of change:** New feature <!-- Use one of the following: Bug fix,
new feature, new example, new tests, documentation. -->

**Overview:** 
- Support the nano and nano-VL in eagle3 online mode:
- Added submodule path detection for `base model`, `lm_head`, and
`embeddings` to adapt different base model naming structure;
  - Refactored data loading/preprocessing to support VLM;
- Attn backend improvement:
  - Added option of `sdpa` in case `flex_attn` doesn't work. 
- Added a unified TTT mask function that produce either `BlockMask` for
flex_attn or tensor masks for regular attn.
 - Logging improvements:
- Added estimated AR validation during training. This is available for
both online and offline.
- Plot estimated AR and training acc to wandb for better training
visualization;
  - Fix: PTQ on speculative decoding model.

## Usage
<!-- You can potentially add a usage example below. -->
For VLM as base model, pass in extra arguments `--vlm_processor
<hf_model_path> --vlm_img_dir <path to images>` in original launching
commands. Other usage unchanged.
E.g. 
```bash
./launch_train.sh --model $MODEL \
            --output_dir $OUTPUT_DIR \
            --data $DATA \
            --num_gpu 1 \
            --num_epochs 2 \
            --train_bs 2 \
            --lr 3e-5 \
            --eagle_config eagle_config.json \
            --training_seq_len 4096 \
            --vlm_processor $MODEL \
            --vlm_img_dir  <path to images>
```
## Testing
<!-- Mention how have you tested your change if applicable. -->
Tested short training with HF Online training on following models:
- `llama-3.2-1b` -  data: daring-anteater
- The new nano (Hyrbid LLM)  - data: daring-anteater
- The nano-VL - data: Llama-Nemotron-VLM-Dataset-v1/ocr_1
 
See loss decreasing and AR > 1.


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`Draft` PR. -->

- **Make sure you read and follow [Contributor
guidelines](https://github.com/NVIDIA/TensorRT-Model-Optimizer/blob/main/CONTRIBUTING.md)**
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- **Is this change backward compatible?**: Yes/No <!--- If No, explain
why. -->
- **Did you write any new necessary tests?**: Yes/No
- **Did you add or update any necessary documentation?**: Yes/No
- **Did you update
[Changelog](https://github.com/NVIDIA/TensorRT-Model-Optimizer/blob/main/CHANGELOG.rst)?**:
Yes/No <!--- Only for new features, API changes, critical bug fixes or
bw breaking changes. -->

## Additional Information
<!-- E.g. related issue. -->

---------

Signed-off-by: h-guo18 <[email protected]>

Author: h-guo18

examples/speculative_decoding/eagle_config.json

@@ -6,5 +6,6 @@
         "original_max_position_embeddings": 8192,
         "rope_type": "llama3"
     },
-    "initializer_range": 0.02
+    "initializer_range": 0.02,
+    "_attn_implementation": "sdpa"
 }