Optimize constructing tensors from external data (#55705)
Summary:
This PR optimizes the way tensors are constructed from external data. It avoids allocating an empty tensor beforehand and directly constructs the target tensor by passing the newly-initialized `DataPtr`. Running some Facebook-internal benchmarks showed that combined with https://github.com/pytorch/pytorch/issues/54530 this PR achieves performance parity with Caffe2 tensor construction. (Overall ~2x speed improvement over the original `at::from_blob()` implementation.)
Testing is done with the existing unit and integration tests as there is no user-observable API change.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55705
Reviewed By: ezyang
Differential Revision: D27686043
Pulled By: cbalioglu
fbshipit-source-id: b365c614476bcf0567797dfaf2add1b76fb6c272
