Make SymInt constructor explicit
Since we plan to have a bunch of code that is sensitive to whether or
not a SymInt contains a symbolic shape or not, it seems like a bad idea
to have an implicit constructor.
For example, code like:
```
sizes_and_strides_.stride_at_unchecked(dim) = 0;
```
would sail through, and the `0` would get implicitly promoted to a
SymInt.
This is a tradeoff though: it makes code that handles `SymInt`s more
clunky as `int64_t`s and integer literals need to be explicitly wrapped
in `SymInt` before being used.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77666
Approved by: https://github.com/ezyang
