Add warning for deprecated to_edge() flow with CoreML

This commit adds a warning message in the CoreML partitioner's partition() method to alert users when they use the older to_edge() flow instead of the recommended to_edge_transform_and_lower() flow.

The warning informs users that:
- Using the old flow may result in performance regression
- The recommended approach is to use to_edge_transform_and_lower() with CoreML partitioner
- Provides a link to the CoreML backend documentation for more details

Fixes #15960

Author: Lokesh9106

backends/apple/coreml/partition/coreml_partitioner.py

@@ -226,6 +226,11 @@ def partition(self, exported_program: ExportedProgram) -> PartitionResult:
         # Run the CapabilityBasedPartitioner to return the largest possible
         # subgraphs containing the nodes with the tags
         logger.info("CoreMLPartitioner::partition")
+                logger.warning("Using the old `to_edge()` flow with CoreML may result in performance regression. "
+            "The recommended flow is to use `to_edge_transform_and_lower()` with the CoreML partitioner. "
+            "See the documentation for more details: "
+            "https://github.com/pytorch/executorch/blob/main/docs/source/backends/coreml/coreml-overview.md#using-the-core-ml-backend"
+        )
         partition_tags = {}
 
         capability_partitioner = CapabilityBasedPartitioner(