Move `Tensor.*` to namespace `api` (#4124)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4124

Then, update usage via
```
find . -type f -exec sed -i 's/vTensor/api::vTensor/g' {} +
find . -type f -exec sed -i 's/api::vTensorPtr/vTensorPtr/g' {} +
```
ghstack-source-id: 232367491
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Reviewed By: SS-JIA

Differential Revision: D59281541

fbshipit-source-id: 2258b9d2da22a966cf5b4c297ba4e5eb47123e0f