PLUGINS/UCX: Improve the performance by specifying memory type in sendAm(). (#2057)

## What?
This PR improves the performance of NIXL by specifying memory type in
sendAm().

## Why?
Each nixlbench batch triggers a notification (in postXfer function). The
notification calls sendAm without providing mh or memory type, in which
case UCX must detect the memory type of the notification buffer in
ucp_memory_detect_internal.

If memtype cache is NULL, ucp_memory_detect_internal enters the slow
path and then queries all possible memory domians, including CUDA
memory, which introduces significant overhead.

Below is the experiment result with block size of 1024B:

| Batch | no-opt BW (GB/s) | opt BW (GB/s) | Gain |

|:-------:|:--------------------------:|:-----------------------:|:-----:|
| 32    | 0.823 | 0.844 | 2.55% |
| 64    | 0.944 | 0.961 | 1.80% |
| 128   | 1.037 | 1.059 | 2.12% |
| 256   | 1.103 | 1.121 | 1.63% |


<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Bug Fixes**
* Improved compatibility and reliability for host-memory data transfers
over UCX.
  * Preserved existing transfer flags and callback behavior.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: ZhenlongMa <zhenlongm@nvidia.com>