feat[cartesian] Literal precision

[twicki]

Description

As we realized there is more mixed-precision work in fv3, this PR allows for generic casts.

Allow for generic casts int and float to a literal precision that is given to any given stencil. This allows for better re-use instead of having to rely on the hard-coded i32 and i64 between stencils.

[romanc]

Looks good to me. Thanks for adding all the docstrings. Some nitpicks about the types.

[stubbiali]

Thank you @twicki, LGTM! I left some minor comments, but the PR could be merged as is.

[stubbiali]

To make it immediately clear that literal_precision can either be 32 or 64, one might define the following type descriptor in gt4py.cartesian.utils.attrib:

def _make_literal_validator(args):
    if not isinstance(args, tuple):
        args = tuple([args])

    def _is_literal_validator(instance, attribute, value):
        if value not in args:
            raise ValueError(
                f"Invalid literal value {value} for attribute `{attribute.name}`; "
                f"expected one of {args}."
            )

    return _is_literal_validator


class _LiteralDescriptor:
    def __getitem__(self, values):
        return _TypeDescriptor(
            "Literal",
            args=values,
            make_validator_func=_make_literal_validator,
            type_hint=typing.Literal,
        )


Literal = _LiteralDescriptor()

so that here we could have:

Suggested change

	literal_precision = attribute(of=int, default=64)
	"Specify the literal precision for automatic casts. Defaults to 64-bit"
	literal_precision = attribute(of=Literal[32, 64], default=64)
	"Specify the literal precision for automatic casts. Defaults to 64-bit"

Just thinking out loud, this comment should not be blocking.

[stubbiali]

Suggested change

	def frontend_type_to_native_type(literal_precision: int = 64) -> dict[str, DataType]:
	def frontend_type_to_native_type(literal_precision: Literal[32, 64] = 64) -> dict[str, DataType]:

[stubbiali]

This check would be redundant with the proposed custom type descriptor for literals.

[stubbiali]

Some tests of this suite are failing for reasons that go beyond this PR. I'm pretty sure you're well aware of this, but would like to double-check anyway.