Configerator based PlanLoader implementation (pytorch#3356)

Summary:
Pull Request resolved: pytorch#3356

Add ConfigeratorPlanLoader an implementation of the PlanLoader interface to enable:

**Key Features:**
1. Plan Retrieval: Loads compressed sharding plans from Configerator using plan_id
2. Database Integration: Queries PlannerStatsDB to get storage location and context hash
3. Decompression: Uses zstd to decompress stored plan data
4. Thrift Conversion: Deserializes Thrift structures and converts back to Python ShardingOption objects
5. Error Handling: Failure scenarios with configurable fallback behavior

**Error Handling & Fallback Scenarios:**

The implementation supports two distinct error handling modes controlled by `enable_fallback`:

**Normal Mode (enable_fallback=False - Default):**
- Raises `PlannerError` with `PLAN_LOADING_FAILED` type for any failure
- Error scenarios include:
  - Network connectivity issues (Configerator service unavailable)
  - Invalid plan id or config path
  - Data decompression failures
  - Thrift deserialization errors
  - Thrift-to-Python conversion failures

**Fallback Mode (enable_fallback=True):**
- Returns `None` instead of raising exceptions on loading failures
- Logs detailed warning messages with plan_id, config_path, and error details
- Enables graceful degradation where system can fall back to alternative planning strategies
- Suitable for development, experimentation, or scenarios prioritizing availability over strict error handling
- Warning logs include full context for debugging: plan ID, Configerator path, and original error

Reviewed By: mserturk

Differential Revision: D81573577

fbshipit-source-id: 93e84c86fb0b9bccd443a93e2b5785e1bc06a349

Author: aliafzal

torchrec/distributed/planner/types.py

@@ -796,6 +796,7 @@ class PlannerErrorType(Enum):
     PARTITION = "partition"
     OTHER = "other"
     PLANNER_INPUT_CONTEXT_MISMATCH = "planner_input_context_mismatch"
+    PLAN_LOADING_FAILED = "plan_loading_failed"
 
 
 class PlannerError(Exception):