[wasm] Fix gather operation bug when using batches

[unexploredtest]

Fixes #8658
Basically the shape of indices is [batchSize, indicesSize / shapeInfo.batchSize], so in order to move to the next batch we have to increment by indicesSize / shapeInfo.batchSize not shapeInfo.batchSize as this code is doing:

loc[2] = indices_ptr[batch_loc * batch_size + indices_loc];

So we simply pass indicesSize / shapeInfo.batchSize instead of shapeInfo.batchSize which is named indicesPerBatch. If the name or coding style is undesired let me know.
Also I wanted to add tests, but I realized there were no units for this kernel... Shall I add one anyways?

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