[dyn.nn.pad] cast pad value to input dtype

[sfvaroglu]

We encountered a bug where dyn.nn.pad did not match input dtype. This PR casts the pad value to input dtype to fix the issue.

[sfvaroglu]

@mbrookhart @AndrewZhaoLuo

[AndrewZhaoLuo]

LGTM, can you write a quick test to catch this?

[sfvaroglu]

LGTM, can you write a quick test to catch this?

I tried to add a test case in relay but ran into other type issues. Turns out pad_value cannot be a constant (expected to be a variable).

However, the problematic op (e.g. dyn.nn.pad(%282, %291, 0f /* ty=float32 */, pad_width=[]) /* ty=Tensor[(?), int64] */) is from a tflite model (tflite->onnx->relay) with a constant pad_value. Perhaps it doesn't go through the same checks.

[AndrewZhaoLuo]

Interesting, hmm ok fine with skipping unittest for now. These can be annoying.