[DECOUPLED-MODE] Checkpointing: import GCS via decoupling stub in dynamic loader

[gulsumgudukbay]

Description

Route the dynamic HuggingFace checkpoint loader's Google Cloud Storage access through the existing decoupling helper instead of importing it directly.

Previously, src/maxtext/checkpoint_conversion/utils/load_dynamic.py did a top-level from google.cloud import storage. In "decoupled" environments (DECOUPLE_GCLOUD=TRUE), where optional Google Cloud dependencies are intentionally not installed, this unconditional import raised ImportError: cannot import name 'storage' from 'google.cloud' at module load.
Because load_dynamic is pulled in transitively by maxtext.common.checkpointing (and thus by maxtext/__init__), the error
aborted the entire pytest collection at tests/conftest.py import time, so the full decoupled test suite failed before running a single test.

The rest of the codebase already accesses GCS through maxtext.common.gcloud_stub.gcs_storage(), which returns the real
google.cloud.storage module when available and a no-op stub otherwise. This PR makes load_dynamic.py follow that same pattern:

• Replace the direct from google.cloud import storage with from maxtext.common.gcloud_stub import gcs_storage.
• Bind storage = gcs_storage() at module scope, so the existing storage.Client() / list_blobs(...) call sites are unchanged.

This is a minimal, low-risk change: in non-decoupled runs behavior is identical (real google.cloud.storage, now with transfer_manager attached by the helper); in decoupled runs the module imports cleanly and GCS calls are stubbed.
No functional GCS behavior changes for real workloads.

Tests

Verified in the ROCm TheRock 7.14 MaxText container with the decoupled venv (DECOUPLE_GCLOUD=TRUE, PYTHONPATH=src):

• Direct import sanity: python -c "import maxtext.checkpoint_conversion.utils.load_dynamic as m; print(m.storage._IS_STUB)"
  imports OK, prints True (stub) in decoupled mode.
• Decoupled pytest collection (previously crashed) now succeeds: pytest -m 'not cpu_only and not tpu_only and not post_training and decoupled' tests --ignore=tests/post_training 2664 collected / 481 deselected / 2 skipped / 2183 selected, no collection error; tests execute normally.

Checklist

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