Feature request: Expose _apply_op as a public method

[SimonBiggs]

Hi,

I have found the private function _apply_op to be quite useful for custom lazy computation. For example:

import ml_collections

structures = ml_collections.FieldReference(
    [
        "parotid_right",
        "parotid_left",
        "brainstem",
        "spinal_cord",
    ],
    field_type=list,
)

################################################
number_of_structures = structures._apply_op(len)
number_of_structures._field_type = int
################################################

edge_length = ml_collections.FieldReference(512, field_type=int)

cfg = ml_collections.ConfigDict(
    {
        "structures": structures,
        "edge_length": edge_length,
        "mask_shape": (edge_length, edge_length, number_of_structures),
    }
)

assert cfg.mask_shape == (512, 512, 4)  # Passes

cfg.structures = ["brainstem", "spinal_cord"]
cfg.edge_length = 64
assert cfg.mask_shape == (64, 64, 2)  # Passes

Might it be possible that this method be exposed as a first class method named apply_op (or more simply apply)? I'd be happy to make a PR including the above as a test if there is interest in making this design adjustment.

Here is an example implementation:

def apply(parent: ml_collections.FieldReference, fn, field_type=None):
    child = parent._apply_op(fn)  # pylint: disable = protected-access

    if field_type is not None:
        child._field_type = field_type  # pylint: disable = protected-access

    a_child_value = child.get()
    if not isinstance(
        a_child_value, child._field_type  # pylint: disable = protected-access
    ):
        raise TypeError(
            f"Expected operation result to be of type {field_type}. "
            f"Instead however it was {type(a_child_value)}. "
            "Adjust the `field_type` parameter accordingly."
        )

    return child

and a corresponding test:

def test_config_apply():
    fr = ml_collections.FieldReference("a,b,c", field_type=str)

    def fn(string):
        return string.split(",")

    with pytest.raises(TypeError):
        apply(fr, fn)

    new_fr = apply(fr, fn, field_type=list)
    assert new_fr.get() == ["a", "b", "c"]

Cheers,
Simon