[gemini] accelerate inference

[ver217]

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1. Accelerate gemini inference by supporting keeping fp16 params gathered in inference mode.
2. Fix chat generation problem. Previous code has compatibility issue and kv cache may not be used.
3. Fix chat opt benchmark.

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[github-actions]

The code coverage for the changed files is 87%.

Click me to view the complete report

Name                                          Stmts   Miss  Cover
-----------------------------------------------------------------
colossalai/zero/gemini/gemini_ddp.py            382     66    83%
tests/components_to_test/__init__.py              4      0   100%
tests/components_to_test/utils/__init__.py        2      0   100%
tests/components_to_test/utils/executor.py       15      1    93%
tests/test_zero/test_gemini/test_fwd_bwd.py     116      1    99%
-----------------------------------------------------------------
TOTAL                                           519     68    87%