Optimize the cat operation on contiguous tensors (#1855)

* Add a specialized kernel for copy2d.

* Move the cat operations.

* Avoid transpositions in cat.

* Bugfix.

* Bugfix for the cuda kernel.

* Add a benchmark.

* Add more testing.

* Test fix.

* Faster kernel.

* Add the missing kernel.

* Tweak the test.

* Add a metal kernel.

* Fix for the metal kernel.

* Get the tests to pass on metal.

* Also use this opportunity to fix the metal kernel for ELU.

* Add some bf16 kernels.

* Clippy fixes.

candle-core/src/lib.rs

@@ -67,6 +67,7 @@ pub mod shape;
 mod storage;
 mod strided_index;
 mod tensor;
+mod tensor_cat;
 pub mod test_utils;
 pub mod utils;
 mod variable;