Make linalg.inv composite of linalg.solve (#80074)
The `getri` kernel calls inside `getrs` so we can do so explicitly
ourselves and save ourselves from having to maintain an extra kernel.
This way we just need to optimise `lu_factor` and `lu_solve` and `inv`
will be as efficient as it can be, as it'll be choosing the best backend
to perform the factorisation and the best backend (not necessarily the
same) to perform the solve.
Fixes https://github.com/pytorch/pytorch/issues/77498
The benchmarks: https://github.com/pytorch/pytorch/pull/80074#issuecomment-1164309071
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80074
Approved by: https://github.com/IvanYashchuk, https://github.com/albanD, https://github.com/malfet
