Fixing tensor.numpy on wrapped tensors

Fixes pytorch#626

Description:
- Fixing tensor.numpy on wrapped tensors

functorch/_src/monkey_patching.py

@@ -98,3 +98,34 @@ def _backward(*args, **kwargs):
 
 
 setattr(torch.Tensor, 'backward', _backward)
+
+
+# Monkeypatch .numpy() to fetch underlying tensor and call .numpy()
+_old_numpy = torch.Tensor.numpy
+
+
+@functools.wraps(_old_numpy)
+def _numpy(tensor):
+    level = _C.maybe_get_level(tensor)
+    if level == -1:
+        return _old_numpy(tensor)
+
+    if _C.is_functionaltensor(tensor):
+        # Since we're unwrapping the FunctionalTensorWrapper, we need to make sure
+        # that it's up to date first
+        torch._sync(tensor)
+
+    value = _C.get_unwrapped(tensor)
+    dl_enabled = _C.tls_set_is_included()
+    try:
+        # Disable temporarily kDynamicLayerFrontModeKey/kDynamicLayerBackModeKey as included dispatch keys
+        if (dl_enabled):
+            _C._set_dynamic_layer_keys_included(False)
+        return value.numpy()
+    finally:
+        # Reenable kDynamicLayerFrontModeKey/kDynamicLayerBackModeKey as included dispatch keys
+        if (dl_enabled):
+            _C._set_dynamic_layer_keys_included(True)
+
+
+setattr(torch.Tensor, 'numpy', _numpy)