Add e5-mistral embedding model - step 3/3

[Ying1123]

This is the last PR contains unit test after

• Add io struct for embedding models [unreachable code] - step 2/3 #987
• Add llama embedding modules [unreachable code] - step 1/3 #983

Note: does not support input is a list of prompts

[merrymercy]

This PR introduces too many if/else. Follow this principle to refactor them

1. If there is a if/else branch, make sure the generation branch is the first one
2. If possible, always use if is_generation instead of if not is_embedding