[torch.Package/TorchScript] TS serialization importer to handle unified format (#54891)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54891
Changed TorchScript's jit/serialization importer logic to handle both original TS serialization format and new unified TS format
Original TS file format:
```
resnet.pt
├── data # tensor data
│ ├── 94286146172688
│ ├── 94286146172784
│ └── ...
├── code/ # TorchScript code
│ ├── __torch__
│ │ ├── torch
│ │ │ └── nn ...
│ │ └── torchvision ...
│ ├── __torch__.py
│ └── __torch__.py.debug_pkl
├── data.pkl # the ScriptModule object, pickled by the TS pickler
├── version # version metadata
├── constants.pkl # any tensor constants present in the TS code
└── extra
├── name_of_file
└── foo
```
Unified file format:
```
─── package_name.pt
├── .data
│ ├── ts_code # code shared between models
│ │ ├── 0
│ │ │ ├── constants.pkl
│ │ │ └── data.pkl
│ │ ├── 1
│ │ │ ├── constants.pkl
│ │ │ └── data.pkl
│ │ └── code
│ │ ├── __torch__
│ │ │ ├── torch
│ │ │ │ └── nn ...
│ │ │ └── torchvision ...
│ │ ├── __torch__.py
│ │ └── __torch__.py.debug_pkl
│ ├── 0.storage
│ ├── 1.storage
│ ├── <many more storages>
│ ├── 201.storage
│ ├── extern_modules
│ └── version
└── res
├── mod.pkl # maps to ts_id 0 and .data/ts_code/0
└── mod2.pkl # maps to ts_id 1 and .data/ts_code/1
```
Test Plan: Imported from OSS
Reviewed By: suo
Differential Revision: D27832548
Pulled By: Lilyjjo
fbshipit-source-id: 4a6e84c3a9bac8eed6a4e4afc2ac76dd691858b0
