[Improve] Improve the perf of roialign with MLU backend

[zcyKTH]

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Motivation

Improve the performance of roialign_forward with MLU backend. The average performance improvement is 48.02%, which measured based on cases generated from Faster R-CNN and Mask R-CNN. The detailed change of performance is listed in the following table.

ID	after (us)	before (us)	(before-after)/before
1	1902	2446	22.24%
2	1448	2494	41.94%
3	3271	4758	31.25%
4	2098	3051	31.24%
5	695	1551	55.19%
6	3449	4895	29.54%
7	63103	773761	91.84%
8	321	1104	70.92%
9	107866	789963	86.35%
10	203	253	19.76%
11	132	211	37.44%
12	1470	2270	35.24%
13	253	405	37.53%
14	283	409	30.81%
15	105382	206562	48.98%
16	75221	381750	80.30%
17	50223	158306	68.27%
18	1487	2530	41.23%
19	743	1022	27.30%
20	967	2797	65.43%
21	1185	3473	65.88%
22	53947	110553	51.20%
23	328	1132	71.02%
24	1151	2188	47.39%
25	2434	4117	40.88%
26	698	2456	71.58%
27	1627	2109	22.85%
28	346	1121	69.13%
29	222	250	11.20%
30	444	1592	72.11%
31	76063	170331	55.34%
32	1496	2096	28.63%
33	27766	273677	89.85%
34	2921	5948	50.89%
35	4285	7484	42.74%
36	1601	2779	42.39%
37	465	1795	74.09%
38	2293	3449	33.52%
39	180	238	24.37%
40	2181	3372	35.32%
41	51097	54585	6.39%
42	1324	4494	70.54%
43	1830	3177	42.40%
44	233	365	36.16%
45	2872	3607	20.38%
46	1139	2895	60.66%
47	161	220	26.82%
48	4564	11816	61.37%
49	1502	2443	38.52%
50	83831	142873	41.32%
51	76	143	46.85%
52	504	774	34.88%
53	224	936	76.07%
54	648	2047	68.34%
55	451	1162	61.19%
56	328	1106	70.34%
57	1218	2043	40.38%
58	253	394	35.79%
59	1250	1922	34.96%
60	351	708	50.42%
61	3901	6529	40.25%
62	629	3477	81.91%
63	2554	5106	49.98%
64	140	183	23.50%
65	3639	6591	44.79%
66	633	1317	51.94%

Modification

Merge all kernels of roialign_forward as one in mmcv/ops/csrc/common/mlu/roi_align_mlu_kernel.mlu.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[teamwong111]

Could you please provide Roialign unit test results in the pr content?

[grimoire]

LGTM. It would be better if you can provide a benchmark about the performance.