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>

docs/source/user_guide/additional_config.md

@@ -39,6 +39,7 @@ The details of each config option are as follows:
 | Name | Type | Default | Description |
 | ---- | ---- | ------- | ----------- |
 | `enabled` | bool | `False` | Whether to enable torchair graph mode |
+| `enable_multistream_mla`| bool | `False` | Whether to put vector ops of MLA to another stream |
 | `enable_multistream_moe`| bool | `False` | Whether to enable multistream shared expert |
 | `enable_view_optimize` | bool | `True` | Whether to enable torchair view optimization |
 | `use_cached_graph` | bool | `False` | Whether to use cached graph |