Consolidate customization contiguous/sizes policy into unified policy
Prior to this PR, we had a mish-mash of ways of getting unconventional
sizes/strides behavior:
- In OSS (but not in fbcode), some methods are virtual and you
can override them directly
- There is a is_contiguous policy which is a bitfield tag that lets
you toggle is_contiguous to error or hit a virtual method
is_contiguous_custom if it is set. Ordinarily is_contiguous()
is virtual and you can just override it, but this works EVEN IF
is_contiguous() is non-virtual (e.g., in fbcode)
- There is also a sizes policy which is the same idea but for sizes
This PR unifies these mechanisms, and in doing so, eliminates the
maybe virtual/not-virtualness of the methods in question. The primary
downside of this change is that it is BC-breaking (but the BC break is
very easy to fix!)
The new scheme works like this: we have three levels of policy for
sizes/strides (order matters).
- The Default policy is a conventional dense tensor, where we use
all of the built-in fields to directly represent the
sizes/strides/numel/contiguity of the tensor, and it is possible
to bypass virtual call entirely.
- The CustomStrides policy represent tensors which have a custom
notion of strides (most typically, that they don't support them),
shunting strides() and is_contiguous() to virtual methods
strides_custom() and is_contiguous_custom(). This INCLUDES handling
for contiguity, since they typically go hand-in-hand (although
the situation is murky with batched tensors). The default
implementations of these functions raise errors saying the tensor
doesn't support them.
- The CustomSizes policy represent tensors which have a custom
notion of sizes (the two notable examples are nested tensor, which
doesn't have a representation of sizes in the conventional form, and
XLA/LTC tensor, which synchronizes its sizes with an underlying
compiler backend). This shunts sizes(), numel() and dim() (along
with everything from strides) to _custom() variants.
There is no special policy for erroring; instead, we just do a vcall
and expect the virtual method to raise an exception (the performance
hit from the vcall doesn't matter because you're about to raise a C++
exception anyway). The default implementations of all overridable
functions are available at _default() which is helpful in some
situations when you just want to do a "sync" and then run the
conventional semantics.
This PR could be extended further in two ways but I did not do them
due to time constraints:
- Ideally, all TENSORIMPL_MAYBE_VIRTUAL would be eliminated from
TensorImpl, by using the same policy trick.
- set_size and set_stride are still virtual; it's not entirely clear
the same trick should be used here though as these methods are
deprecated.
Signed-off-by: Edward Z. Yang <ezyangfb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77036
Approved by: https://github.com/bdhirsh