This feature had been marked as `TODO` in the `tensor.splat`
documentation for a while. This MR includes:
- Support for dynamically shaped tensors in the return type of
`tensor.splat` with the syntax suggested in the `TODO` comment.
- Updated op documentation.
- Bufferization support.
- Updates in op folders affected by the new feature.
- Unit tests for valid/invalid syntax, valid/invalid folding, and
lowering through bufferization.
- Additional op builders resembling those available in `tensor.empty`.
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