Remove unecessary copy within blob serialization (#40096)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40096
Declaring `tensor_proto` to be of type `auto` means that it will copy the entire `TensorProto` instead of just keeping a reference. This changes it to just use a const reference instead.
Test Plan:
Using the model loader benchmark to measure model loading performance:
### `tensor_proto` is of type `const auto&`
```
============================================================================
caffe2/caffe2/fb/predictor/ModelLoaderBenchmark.cpprelative time/iter iters/s
============================================================================
BlobProtoInt32DeserializationFloat16 11.08ms 90.27
BlobProtoByteDeserializationFloat16 1509.73% 733.73us 1.36K
----------------------------------------------------------------------------
BlobProtoInt32DeserializationUInt8 10.48ms 95.45
BlobProtoByteDeserializationUInt8 2974.57% 352.22us 2.84K
============================================================================
```
### `tensor_proto` is of type `auto`
```
============================================================================
caffe2/caffe2/fb/predictor/ModelLoaderBenchmark.cpprelative time/iter iters/s
============================================================================
BlobProtoInt32DeserializationFloat16 13.84ms 72.26
BlobProtoByteDeserializationFloat16 658.85% 2.10ms 476.08
----------------------------------------------------------------------------
BlobProtoInt32DeserializationUInt8 17.09ms 58.51
BlobProtoByteDeserializationUInt8 3365.98% 507.80us 1.97K
============================================================================
```
Reviewed By: marksantaniello
Differential Revision: D21959644
fbshipit-source-id: 6bc2dfbde306f88bf7cd4f9b14b95ac69c2e1b4d
