[ATen] Remove `AT_ASSERTM` from Blob::free_() (#33929)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33929
`Blob::~Blob()` calls `Blob::free_()`. `Blob::free_()` throws and destructors should not throw.
A few other minor tweaks include:
- Remove `static_cast<void*>()` in `ShareExternal`
- Remove default values of `pointer_` and `has_ownership_`
Test Plan:
```
buck test caffe2/caffe2:caffe2_test_cpu
```
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Reviewed By: yinghai
Differential Revision: D19153199
fbshipit-source-id: f93983d5bf324b9a464ad2d1ed0dba13f807d2f6
