onnxruntime

[js/webgpu] fix buffer size when download (#15990)

### Description
fix buffer size when download. buffer size should always be padded to
multiple of 4.

resolved issue described in #15796

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![Image](https://user-images.githubusercontent.com/26504141/239093785-9417dffc-6f00-47b2-956d-402b43bdb0a9.png)