copy_from() crashes for fields of struct types

[AmesingFlank]

The following code crashes

@ti.dataclass
class C:
    i: int
    f: float

cf1 = C.field(shape = (2,))
cf2 = C.field(shape = (2,))

cf1.copy_from(cf2)

with the error message

TypeError: Cannot copy from a non-field object

Interestingly, if the copy is performed on 2D fields, the crash has a different error message:

@ti.dataclass
class C:
    i: int
    f: float

cf1 = C.field(shape = (2,2))
cf2 = C.field(shape = (2,2))

cf1.copy_from(cf2)

AssertionError: Slicing is not supported on ti.field