Fix quantization test failure

Summary:
# Context
In post training quantization callback, we make a deepcopy of the Lightning module before validation start and prepare the copy with FX quantization API. The callback keeps the prepared model inside it.

# The problem
By the second time we run the validation epoch, we try to make a copy of the Lightning module, which has a reference to trainer, which has a reference to quantization callback, which has a prepared model, which is not deepcopiable.

# Mitigation
Delete the trainer before making a deepcopy.
We're already doing that in stl/callbacks/quantization, but the changes were not ported into D2 (https://github.com/facebookresearch/d2go/commit/4169abc18ec539a24081b179fcbbc5a5754d102b)Go.

Reviewed By: zhanghang1989

Differential Revision: D29409085

fbshipit-source-id: 24550124181673b2e567b2a04563bcdfb440e145

d2go/runner/callbacks/quantization.py

@@ -54,18 +54,14 @@ def rhasattr(obj: Any, attr: str, *args) -> bool:
 
 
 def _deepcopy(pl_module: LightningModule) -> LightningModule:
-    """Copy a LightningModule. Some properties need to be ignored. """
-    # Remove _result before call to deepcopy since it store non-leaf Tensors.
-    # If not removed, you'll see this error on deepcopy() attempts: P150283141.
-    if hasattr(pl_module, "_results"):
-        result = pl_module._results
-        delattr(pl_module, "_results")
-        copy = deepcopy(pl_module)
-
-        # Set back.
-        pl_module._results = result
-    else:
+    """Copy a LightningModule. Some properties need to be ignored."""
+    # Remove trainer reference.
+    trainer = pl_module.trainer
+    try:
+        pl_module.trainer = None
         copy = deepcopy(pl_module)
+    finally:
+        pl_module.trainer = trainer
     return copy