quantity_input could provide access to the unwrapped function

Description

It is very handy to use the quantity_input decorator to ensure the inputs to a function are Quantities with appropriate units. But this checking makes it impossible to use the function with the python package uncertainties or other automatic-differentiation tools that rely on passing things that behave enough like numbers to pass through simple mathematics. In an ideal world one would be able to attach units to such exotic objects, but in practical terms the easiest thing to do is to pass these objects through the raw un-decorated version of the function - if the decorator provided that.

Additional context

I suggest usage that looks like:

@quantity_input
def f(x: u.m):
    return x**2

f(1*u.m)

f.raw(ufloat(1, 0.1))

This should be implementable, if desired, just by storing the function to be wrapped in an attribute of the wrapper object.

[mhvk]

Slightly off topic, but since uncertainties was mentioned: I’ve long meant to have a proper Measurement class in astropy.uncertainties, and got quite a bit of the way to make an array-based version of uncertainties - see #3715 for an initial implementation and discussion about pitfalls as well as the current version at https://github.com/astropy/astropy/compare/main...mhvk:uncertainty?expand=1

If I remember correctly, yhe main problem is tracking covariances for operations involving non-trivial broadcasts. I may get back to that once Masked is really fully functional…

[aarchiba]

We had some discussion that the documentation for quantity_input wasn’t as friendly and discoverable as we wished, so a PR that fleshed it out and mentioned the availability of __wrapped__ would be nice. My schedule is a little full just now, though.