fix: convert string values to enum types in update_config

[Mr-Neutr0n]

Summary

Fix KeyError when passing enum values like sharding_strategy as strings from the command line.

Problem

When running with --fsdp_config.sharding_strategy "FULL_SHARD", the string value is not converted to ShardingStrategy enum, causing:

KeyError: 'FULL_SHARD'
# at torch/distributed/fsdp/_init_utils.py
# SHARDING_STRATEGY_MAP[state.sharding_strategy]

PyTorch's FSDP expects a ShardingStrategy enum value, not a string.

Solution

Added _convert_to_field_type() helper function that:

1. Uses get_type_hints() to get the expected field type from the dataclass
2. If the expected type is an Enum and the value is a string, converts it using EnumClass[value]
3. Returns the original value if conversion fails or is not needed

This allows users to pass enum values as strings from the command line:

--fsdp_config.sharding_strategy FULL_SHARD
--fsdp_config.sharding_strategy HYBRID_SHARD
--fsdp_config.checkpoint_type SHARDED_STATE_DICT

The fix also works for checkpoint_type (StateDictType enum) and any future enum fields.

Fixes #809

[Mr-Neutr0n]

Friendly bump! Let me know if there's anything I should update or improve to help move this forward.