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

[claydugo]

Problem

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

This line indicated to me the import was intended to be deferred.

Steps to Reproduce

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 dmlc/decord#362 and related packaging issues conda-forge/staged-recipes#31532