[FSDP] Validate activation checkpointing auto-wrap policy

[sdjasj]

What does this PR do?

Rejects FSDP activation checkpointing configurations that use NO_WRAP and raises an actionable ValueError during plugin initialization.

Activation checkpointing selects modules through the configured auto-wrap policy. NO_WRAP is normalized to None, so the previous code failed later in model preparation with an opaque TypeError: 'NoneType' object is not callable.

The regression coverage exercises both environment-variable configuration and direct FullyShardedDataParallelPlugin construction for FSDP1 and FSDP2.

Fixes #3822

Validation

python -m pytest -q \
  tests/fsdp/test_fsdp.py::FSDPPluginIntegration::test_activation_checkpointing_requires_auto_wrap_policy \
  tests/fsdp/test_fsdp.py::FSDP2PluginIntegration::test_activation_checkpointing_requires_auto_wrap_policy
# 2 passed

ruff check src/accelerate/utils/dataclasses.py tests/fsdp/test_fsdp.py
ruff format --check src/accelerate/utils/dataclasses.py tests/fsdp/test_fsdp.py