dynamo: support attribute access on tensor subclasses without sources (#117666)
Fixes https://github.com/pytorch/pytorch/issues/117596
This was needed for Float8Tensor. Before this PR, dynamo would sometimes handle attribute access on tensor subclasses correctly, but it would choke on tensor subclasses with no source (it would fall back to using a `GetAttrVariable` to represent the attribute access, which is a problem if the attribute is a tensor that we later want to call tensor methods on).
I supported two cases:
(1) the attribute is a tensor, which is part of the `attrs` returned by the subclass's `__tensor_flatten__`. This creates a `TensorVariable`
(2) the attribute is a constant, which is part of the constant metadata returned by `__tensor_flatten__`. As per the contract of tensor_flatten, this should be a `ConstantVariable`. It could be possible that we allow non-constant metadata in the future, but we don't support that today.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/117666
Approved by: https://github.com/zou3519
ghstack dependencies: #117667
