Retainable is no more (pytorch#11900)

Summary:
Stack:
    :black_circle:  **pytorch#11900 Retainable is no more**  [:yellow_heart:](https://our.intern.facebook.com/intern/diff/D9977505/)
    :white_circle:  pytorch#11902 Refactor fastGet/fastSet for clarity, removing a null pointer check.  [:yellow_heart:](https://our.intern.facebook.com/intern/diff/D9977654/)

Kill it with fire
Pull Request resolved: pytorch#11900

Differential Revision: D9979779

Pulled By: ezyang

fbshipit-source-id: 0a437e7a0baadb6440e7dc39a01b4a406171faa7

Author: ezyang

aten/src/ATen/Retainable.h

@@ -327,10 +327,10 @@ struct PreprocessGraph {
 
 };
 
-// previously the interpreter worked with at::Retainable values,
-// which are annoying to handle since 99% of values are at::Tensor anyway
-// instead we create a fake subclass of TensorImpl that can be subclassed
-// to hold arbitrary things
+// Sometimes we want to pass things that are not tensors.  Instead of
+// coming up with some "superclass" for tensor, which is annoying since
+// 99% of values are at::Tensor, we instead we create a fake subclass of
+// TensorImpl that can be subclassed to hold arbitrary things
 // Note: this is currently unused but will probably be useful in the future,
 // so we keep it around
 struct ContainerTensor : public at::TensorImpl {