[BE][FSDP] Retire `_get_full_detached_param()` (#80871)

The tests did not actually require that the parameters be detached, so this coalesces `_get_full_detached_param()` with `get_full_params()`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80871
Approved by: https://github.com/rohan-varma
diff --git a/test/distributed/fsdp/test_fsdp_state_dict.py b/test/distributed/fsdp/test_fsdp_state_dict.py
index 367f5cb..0ef5c15 100644
--- a/test/distributed/fsdp/test_fsdp_state_dict.py
+++ b/test/distributed/fsdp/test_fsdp_state_dict.py
@@ -29,7 +29,6 @@
 from torch.testing._internal.common_fsdp import (
     FSDPTest,
     get_full_params,
-    _get_full_detached_param,
     _get_state_dict,
     SkipModel,
     _zero_model,
@@ -350,7 +349,7 @@
         )
         model = self._get_simple_nested_model(mixed_precision=mixed_precision)
         optim = torch.optim.SGD(model.parameters(), lr=0.1)
-        initial_params = _get_full_detached_param(model)
+        initial_params = get_full_params(model)
         for _ in range(6):
             inp = torch.randn(1, 10, device=torch.cuda.current_device())
             output = model(*inp)
@@ -360,7 +359,7 @@
             loss.backward()
             optim.step()
 
-        trained_params = _get_full_detached_param(model)
+        trained_params = get_full_params(model)
         # Ensure some training occured
         self.assertNotEqual(initial_params, trained_params)
         # Save a copy of the state_dict
@@ -392,7 +391,7 @@
 
         with FSDP.state_dict_type(model, STATE_DICT_MAPPING[state_dict_type]):
             model.load_state_dict(state_dict)
-        loaded_params = _get_full_detached_param(model)
+        loaded_params = get_full_params(model)
         self.assertEqual(loaded_params, trained_params)
 
     def _initialize_model(
diff --git a/torch/testing/_internal/common_fsdp.py b/torch/testing/_internal/common_fsdp.py
index 15fa53b..b529c80 100644
--- a/torch/testing/_internal/common_fsdp.py
+++ b/torch/testing/_internal/common_fsdp.py
@@ -30,11 +30,6 @@
     # Don't move model to CUDA at all.
     CUDA_NEVER = 3
 
-def _get_full_detached_param(fsdp_model: FullyShardedDataParallel):
-    with FullyShardedDataParallel.summon_full_params(fsdp_model):
-        params = list(p.clone().detach_() for p in fsdp_model.parameters())
-
-    return params
 
 def _validate(model, process_group, assert_fn):
     module_states = [param.detach().cpu() for param in model.parameters()]