Support multistream of MLA vector operations (#1135)

### What this PR does / why we need it?
Move all vector operations to a secondary stream, with the expected
overlaping being:
```
              | q_rmsnorm |                  | kv_norm_rope_cache |       | q_rope |
| matmul W_DQ | matmul W_DKV | index | index |    matmul W_UQ     | split | matmul W_KV_T |
```

Currently, the `IndexByTensor` operators introduced by computation of
`cos` and `sin` can't be offloaded to the secondary stream due to a
known bug of graph fusion optimization pass. So we instead keep it in
the main stream, only requires it be computed before `matmul W_UQ` to
avoid hindering later overlapping. The problem may be solved by later
optimization (#993), which hoists the computation of `cos` and `sin` up
to the first layer.

### Does this PR introduce _any_ user-facing change?
Controlled by `torchair_graph_config.enable_multistream_mla`, defaulted
to False.

### How was this patch tested?
Tested on 1x16 910 node, with tailored 2 layer DSKv2.

Signed-off-by: sdmyzlp <lrwei2@petalmail.com>

tests/singlecard/test_ascend_config.py

@@ -59,6 +59,7 @@ def test_run_with_ascend_config():
             "graph_batch_sizes": [1, 2, 4, 8],
             "graph_batch_sizes_init": False,
             "enable_multistream_moe": True,
+            "enable_multistream_mla": True,
         },
         "ascend_scheduler_config": {
             "enabled": True,
@@ -79,6 +80,7 @@ def test_run_with_ascend_config():
             1, 2, 4, 8
         ]
         assert not ascend_config.torchair_graph_config.graph_batch_sizes_init
+        assert ascend_config.torchair_graph_config.enable_multistream_mla
         assert ascend_config.torchair_graph_config.enable_multistream_moe
         assert ascend_config.ascend_scheduler_config.enabled
         assert ascend_config.ascend_scheduler_config.enable_chunked_prefill