don't instantiate instance if not needed in sensor execution (#25429)

## Summary & Motivation

We only use the CachingDynamicPartitionsLoader when there's a run request with partitions, and the CachingDynamicPartitionsLoader needs to hydrate the instance ref which is costly. to speed up the typical case, use @functools.cache to memoize the creation of the `CachingDynamicPartitionsLoader(context.instance)`

## How I Tested These Changes

Ran 10k no-op sensor executions before and after the change, this dropped the latency from 20s to 2s

python_modules/dagster/dagster/_core/definitions/sensor_definition.py

@@ -1,3 +1,4 @@
+import functools
 import inspect
 import logging
 from collections import defaultdict
@@ -243,6 +244,10 @@ def resource_defs(self) -> Optional[Mapping[str, "ResourceDefinition"]]:
     def sensor_name(self) -> str:
         return check.not_none(self._sensor_name, "Only valid when sensor name provided")
 
+    @functools.cached_property
+    def caching_dynamic_partitions_loader(self):
+        return CachingDynamicPartitionsLoader(self.instance) if self.instance_ref else None
+
     def merge_resources(self, resources_dict: Mapping[str, Any]) -> "SensorEvaluationContext":
         """Merge the specified resources into this context.
 
@@ -1030,10 +1035,6 @@ def _get_repo_job_by_name(context: SensorEvaluationContext, job_name: str) -> Jo
                 *_run_requests_with_base_asset_jobs(run_requests, context, asset_selection)
             ]
 
-        dynamic_partitions_store = (
-            CachingDynamicPartitionsLoader(context.instance) if context.instance_ref else None
-        )
-
         # Run requests may contain an invalid target, or a partition key that does not exist.
         # We will resolve these run requests, applying the target and partition config/tags.
         resolved_run_requests = []
@@ -1072,7 +1073,7 @@ def _get_repo_job_by_name(context: SensorEvaluationContext, job_name: str) -> Jo
                     run_request.with_resolved_tags_and_config(
                         target_definition=selected_job,
                         current_time=None,
-                        dynamic_partitions_store=dynamic_partitions_store,
+                        dynamic_partitions_store=context.caching_dynamic_partitions_loader,
                         dynamic_partitions_requests=dynamic_partitions_requests,
                     )
                 )