Metal random-generation bug fixes (#1811)

* use_resource API misunderstood. It is not additive. Several usages must be bit-ORed together.

* The seeding was incorrect and used the address instead of the value of the passed in seed.

* Add a check that likely exhibits failure to update the seed between generation of random tensors.

* Buffer overrun, the length given to the std::ptr::copy call was in bytes, and not 32-bit units.

* By default seed the RNG with a time-based value, so that different runs may produce different output, just like the CPU engine.
Use device.set_seed if determinism is warranted.

* Revert "By default seed the RNG with a time-based value, so that different runs may produce different output, just like the CPU engine. Use device.set_seed if determinism is warranted."

This reverts commit d7302de9

Discussion in https://github.com/huggingface/candle/pull/1811#issuecomment-1983079119

* The Metal random kernel failed to set element N/2 of tensors with N elements, N being even.  The reason was that all threads but thread 0 all created 2 random samples, but thread 0 only one, i.e. an odd number.  In order to produce an even number of samples, the early termination of thread 0 should only everr occur for odd sized tensors.

* Add a test catching any deterministic tensor element in rand and randn output.

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Co-authored-by: niklas <niklas@appli.se>
Co-authored-by: Ivar Flakstad <69173633+ivarflakstad@users.noreply.github.com>

candle-core/src/metal_backend.rs

@@ -1727,7 +1727,7 @@ impl BackendDevice for MetalDevice {
         let seed_buffer = self.seed.try_lock().map_err(MetalError::from)?;
         let contents = seed_buffer.contents();
         unsafe {
-            std::ptr::copy([seed].as_ptr(), contents as *mut u32, 4);
+            std::ptr::copy([seed].as_ptr(), contents as *mut u32, 1);
         }
         seed_buffer.did_modify_range(metal::NSRange::new(0, 4));