[BugFix] Fix a problem of failing to utilize custom kv cache dtype.

[whx-sjtu]

This PR fixes bugs in FA3 quantization situation.
There are three problems in total:

1. Not use self.kv_cache_dtype to initialize kv_cache_spec in model_runner_v1.py, fixed.
2. In v0 situation, if we pass "kv_cache_dtype" to initialize vllm, program will collapse due to a validation in vllm's config.py (https://github.com/vllm-project/vllm/blob/c1e4a4052d65d72d45e39db1edb6b7deb4ffd426/vllm/config.py#L1496)
3. In v1 situation, currenly "kv_cache_dtype" can't be passed as described in issue 17355

To solve problem 2 and 3, we currently pass kv_cache_dtype through additional_config and set the cache dtype of cache_config in platform's check_and_update_config
cc @wangxiyuan