Enable packed depthwiseConv for arbitrary dilation / padding.

[annxingyuan]

Note that no tests were added on account of #1469

This change is 

[dsmilkov]

Reviewed 2 of 2 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @annxingyuan, @dsmilkov, and @nsthorat)

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src/kernels/webgl/conv_packed_gpu_depthwise.ts, line 55 at r1 (raw file):

    /**
     * This vectorized implementation of depthwiseConv works by gathering the

is this explanation no longer relevant? can you add another one in the new code

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src/kernels/webgl/conv_packed_gpu_depthwise.ts, line 95 at r1 (raw file):

                }
              `;
            } else {

Since you have long if/else branches, add a comment on this else, here and on the other else`s