Rename depth to channels

[primenumber]

What this patch does to fix the issue.

there are several naming for 'number of channels in a layer: depth channel channels
This PR unify to channels.
kernel_depth (for convolution) -> input_channels
depths -> channels_list
channel -> channels
depth -> channels

Link to any relevant issues or pull requests.

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[a-hanamoto]

RA (reverse-shadowing)

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[iizukak]

OA

[iizukak]

/ready

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[tkng]

RA

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/ready

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[iizukak]

/ready

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