Turn on BUILD_NAMEDTENSOR permanently (#26060)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26060
This PR enables BUILD_NAMEDTENSOR by default. This is done via including
a header, `c10/core/EnableNamedTensor`, that sets `BUILD_NAMEDTENSOR`.
In the future, the plan is to get rid of the flag entirely: we can
incrementally delete usages after this PR goes in.
This PR also maintains the namedtensor ci vs regular ci distinction.
`test/test_namedtensor.py` only runs if TEST_NAMEDTENSOR=1 is specified.
TEST_NAMEDTENSOR=1 is set on the namedtensor ci. I'll remove this
distinction later and send out an announcement about it; devs will be
responsible for named tensor failures after that.
The initial reason why we had the BUILD_NAMEDTENSOR flag was so that we
could quickly prototype named tensor features without worrying about
adding overhead to the framework. The overheads can be categorized as
memory overhead and performance overhead.
Memory overhead: named tensors adds 1 additional word per Tensor. This
is because TensorImpl stores a `unique_ptr<NamedTensorMetaInterface>`
field. This is not a lot of overhead.
Performance overhead: At all entry points to name inference, we check
if inputs to an op are named. If inputs are not named, we short-circuit
and don't do name inference. These calls should therefore be as
efficient as error-checking code and not take up a lot of time.
My plan is to benchmark a few functions and then post the results in a
comment to this PR.
Test Plan: - [namedtensor ci]
Differential Revision: D17331635
Pulled By: zou3519
fbshipit-source-id: deed901347448ae2c26066c1fa432e3dc0cadb92
