fix(template): keep MiMo dependencies optional

[RerankerGuo]

PR type

• Bug Fix
• New Feature
• Document Updates
• More Models or Datasets Support

PR information

MiMo-V2.5 support added qwen_vl_utils as a module-level import in
swift.template.templates.mimo. Because all template modules are registered
when swift.template is imported, environments without that model-specific
optional dependency could no longer import unrelated template, dataset,
inference, trainer, or export modules.

This is visible in the NPU job for #9880, where 34 test modules failed during
collection with:

ModuleNotFoundError: No module named 'qwen_vl_utils'

The dependency is already declared in the MiMo model's requires list. This
change follows the existing Qwen, Llava, Dots, and Valley template pattern by
importing fetch_image and fetch_video only when MiMo media is actually
processed. MiMo users still receive the dependency error at the relevant
feature boundary, while unrelated imports remain available.

A subprocess regression test explicitly blocks every qwen_vl_utils import
and verifies that swift.template still imports successfully.

Experiment results

Before:

.venv/bin/python -m unittest tests.general.test_optional_template_dependencies
ModuleNotFoundError: No module named 'qwen_vl_utils'
FAILED (errors=1)

After:

.venv/bin/python tests/run.py \
  --test_dir tests/general \
  --pattern test_optional_template_dependencies.py
SUCCESS (Runs=1, success=1)

.venv/bin/python -m unittest tests.general.test_model
Ran 1 test
OK

.venv/bin/pre-commit run --all-files
All hooks passed

The local environment does not have qwen-vl-utils installed, so the direct
swift.template, swift.infer_engine.protocol, and swift.dataset imports
also verify the real missing-dependency path.

[RerankerGuo]

CI note: the new optional-dependency regression test passes in the CUDA run
(test_template_import_without_qwen_vl_utils, 7.8s). The run's only error is
the repository-wide ModuleNotFoundError: transformers.models.gemma3 in
test_gemma3_template.py, after the shared environment dependency mutation
already traced on #9862:
#9862 (comment).
Lint passes, and the NPU unittest is still running.