[v1] Refactor KVCacheConfig

[heheda12345]

This PR makes the following changes to KVCacheConfig

1. change the meaning of KVCacheSpec from the spec of the model to the spec of one layer
2. KVCacheConfig class:
   1. change the concept of "group" to "virtual_layer"
   2. save the kv_cache_spec for each virtual_layer instead of each layer
3. The logic to make the same KVCacheConfig for all workers (EngineCore._initialize_kv_caches). Change it into 3 steps:
   1. get the available memory of each worker independently
   2. get the kv_cache_config for each worker independently
   3. adjust the kv_cache_config for all workers to make them the same, including assigning the same num_blocks and the same virtual layers (make_kv_cache_configs_consistent)

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