Fix GroupNorm dynamic input

[sercand]

Fixes #1303 for GroupNorm only.

Added a test case for this issue. Without this patch the test fails with

cls = <class 'coremltools.converters.mil.mil.builder.Builder'>, val = [6, 1, 1, is28, is29], name = 'reshape_14_shape', before_op = None

    @classmethod
    def _add_const(cls, val, name, before_op):
        if not is_python_value(val):
>           raise ValueError("Cannot add const {}".format(val))
E           ValueError: Cannot add const [6, 1, 1, is28, is29]

coremltools/converters/mil/mil/builder.py:76: ValueError

[YifanShenSZ]

Thank @sercand for the great PR! This PR inspires me of the culprit to issues #1303 and #1921. I opened issue #1926 to see if we can have a general solution to all of them

[YifanShenSZ]

CI: https://gitlab.com/coremltools1/coremltools/-/commit/5be49ecec4842e7551ef306f748e67b7ac8b8b12/pipelines

Will approve and merge once CI turns green

[YifanShenSZ]

Got an CI error unrelated to this PR, same as the one in PR #1923

[jakesabathia2]

@sercand Thanks for your contribution to coremltools!