Executorch Model Size Analysis Tool (#68)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/68

Adds a Size Analysis Tool for Executorch models, which provides information about tensor data and delegate blobs in a model. It can be used by calling ```generate_model_size_information``` directly, or from the command line
```
buck2 run executorch/sdk/size_analysis_tool:size_analysis_tool -- --etrecord_path [...] --output_path [...]
```

Reviewed By: mcr229

Differential Revision: D48255922

fbshipit-source-id: 2d68bcb86ea347d82435bce095bc0669d3e889b5