[js/webgpu] Optimize ConvTranspose (Continue) (#23429)
BUG #23273
This PR does below optimizations:
1. When output channels is one, 1) calculate the offset before the
inchannel loop to reduce indices to offsets calculation, 2) split the
`inputChannelsPerGroup` into `inputChannelsPerGroupInt` and
`inputChannelsRemainder` parts so that we can always access 4 data for
`inputChannelsPerGroupInt`.
2. Use precise initial value to reduce useless loop iterations. Thanks
@jiangzhaoming 's suggestion's on this.
With this PR, ConvTranspose becomes 3.7s from 8.4s on Intel Meteor Lake.
On NV RTX 2000 Ada, it becomes 1.6s from 2.7s.
