Replace `sympy.solve` with a new simplified one. (#105877)
This PR implements `try_solve`: a function that tries to move terms of a relational
expression around, so as to isolate a given variable on the left-hand side.
For example:
```python
>>> try_solve(Eq(a + 5, 3), a)
Eq(a, -2)
>>> try_solve(Gt(Mod(a, 3), 0), a) # returns None
>>> try_solve(Gt(Mod(a, 3), 0), Mod(a, 3))
Gt(Mod(a, 3), 0), Mod(a, 3)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105877
Approved by: https://github.com/ezyang
