Update docs of saved_tensors_hooks to avoid ref cycle (#153049)

Fixes #115255

Pull Request resolved: https://github.com/pytorch/pytorch/pull/153049
Approved by: https://github.com/Skylion007, https://github.com/soulitzer

docs/source/notes/autograd.rst

@@ -793,7 +793,7 @@ Example:
 
     def pack_hook(x):
         if x.numel() < SAVE_ON_DISK_THRESHOLD:
-            return x
+            return x.detach()
         temp_file = SelfDeletingTempFile()
         torch.save(tensor, temp_file.name)
         return temp_file
@@ -833,7 +833,7 @@ Tensor object creation. For example:
 
 .. code::
 
-    with torch.autograd.graph.saved_tensors_hooks(lambda x: x, lambda x: x):
+    with torch.autograd.graph.saved_tensors_hooks(lambda x: x.detach(), lambda x: x):
         x = torch.randn(5, requires_grad=True)
         y = x * x
 

torch/autograd/graph.py

@@ -272,7 +272,7 @@ class saved_tensors_hooks:
         >>> # xdoctest: +REQUIRES(env:TORCH_DOCTEST_AUTOGRAD)
         >>> def pack_hook(x):
         ...     print("Packing", x)
-        ...     return x
+        ...     return x.detach()
         >>>
         >>> def unpack_hook(x):
         ...     print("Unpacking", x)
@@ -295,6 +295,11 @@ class saved_tensors_hooks:
     .. warning ::
         Only one pair of hooks is allowed at a time. When recursively nesting this
         context-manager, only the inner-most pair of hooks will be applied.
+
+    .. warning ::
+        To avoid reference cycle, the return value of ``pack_hook`` cannot hold a
+        reference to the input tensor. For example, use `lambda x: x.detach()`
+        instead of `lambda x: x` as the pack hook.
     """
 
     def __init__(