[pt2] Fix arange to match ATen behavior (#93353)
Fixes #92676
`arange` infers the output dtype from the argument types, but in order to reduce
falling back to ATen, inductor preferred to cast whole number float arguments to
int which gave the wrong output dtype. Instead, this decomposes floating point
arange into the prim equivalent for integers.
This also changes the signature of `prims.arange` to
```python
prims.iota(length, *, start, step, **factory_kwargs)
```
which only supports integers arguments. This is done because calculating the
output size from `start, end, step` is surprisingly complex and liable to off by
one errors so should not be duplicated in each backend.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93353
Approved by: https://github.com/ngimel, https://github.com/lezcano
