Fix `FIXME_default_names` by storing static list of 64 none names (#24885)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24885
Store a static pre-allocated vector of names. When one calls
`default_names`, it gives a const reference to some amount of these
names.
Also make clearer the maximum number of dimensions we support for named
tensors. Right now it is 64 but that number is easy to change. 64
follows some internal pytorch maximum number of dimensions;
TensorIterator reduce ops have a limit of 64 dims.
Test Plan: - new tests [namedtensor ci]
Differential Revision: D16915803
Pulled By: zou3519
fbshipit-source-id: 931741b199456f8976882b82f25ab5af6dcd108b
