Input shape bug in DepthwiseConv2D in NCHW format

[DwayneDuane]

In the DepthwiseConv2dNative method, there appears to be a hard coded assumption of channel_last ordering when extracting the channel depth of the input node. This causes conversion to fail in channel_first mode. For instance, in channel_first mode x.shape can evaluate to (is0, 48, is5, is6), and C_in gets assigned is6, which causes a crash later.

[aseemw]

Hi @DwayneDuane , thanks for the change. However, I see that its causing failures on existing tests.
Also , could you add a unit test, that fails without this change but passes with it?

[DwayneDuane]

@aseemw I'm not an expert of Gitlab CI/CD, but the unit test errors do not appear to be related to my changes. I see errors like:
InvalidArchiveError('Error with archive /Users/gitlab/miniconda3/pkgs/ca-certificates-2020.10.14-0.conda. You probably need to delete and re-download or re-create this file. Message from libarchive was:\n\nFile is not a zip file')
or
Reinitialized existing Git repository in /Users/gitlab/builds/esohtQ6e/0/zach_nation/coremltools/.git/ fatal: the remote end hung up unexpectedly
Looks more like a network or CI/CD pipeline setup problem.

[aseemw]

I see errors that seem related to this change as well:
in TestSeparableConv

469        if C_in % groups != 0:
470            msg = "# of input channels {} not divisible by groups {}"
471>           raise ValueError(msg.format(C_in, groups))
472E           ValueError: # of input channels 2 not divisible by groups 12
473../../envs/coremltools-py2.7/lib/python2.7/site-packages/coremltools/converters/mil/mil/ops/defs/conv.py:116: ValueError

[tdp2110]

I don't know what's possible, but if data_format is neither of "NHWC" or "NCHW", then C_in will be an unbound variable