Eager import of `decord` during `import sam3` causes unnecessary dependency and crashes

## Problem

When importing sam3 (`import sam3`), the `decord` library is eagerly loaded even though it's only needed for video file processing.

[This line](https://github.com/facebookresearch/sam3/blob/84cc43bca4347b772f17/sam3/model/utils/sam2_utils.py#L213) indicated to me the import was intended to be deferred.

## Steps to Reproduce

```python
import sys
import sam3

print('decord loaded:', any('decord' in m for m in sys.modules))
# Output: decord loaded: True
```

## Root Cause

The import chain is:
```
import sam3
└─ sam3/model_builder.py
└─ sam3/model/sam3_image.py:17
└─ from sam3.train.data.collator import BatchedDatapoint
└─ sam3/train/data/sam3_image_dataset.py:18
└─ from decord import cpu, VideoReader # Eager import
```

## Impact

See https://github.com/dmlc/decord/pull/362 and related packaging issues https://github.com/conda-forge/staged-recipes/pull/31532

Start with the import chain in sam3/model_builder.py, sam3/model/sam3_image.py, and sam3/train/data/sam3_image_dataset.py, especially the decord import noted in the issue; compare it with the deferred-import intent in sam3/model/utils/sam2_utils.py. Run the documented `import sam3` reproduction and confirm decord is not loaded during import while video processing still has access to it.

Written by the indexing model from the issue text.