Add truncation to CLIP model

[MrLoh]

It seems that sentence transformer does currently not support truncation on CLIP models, which leads to an error when calling SentenceTransformer("sentence-transformers/clip-ViT-L-14").encode("my long text" * 26)

Token indices sequence length is longer than the specified maximum sequence length for this model (81 > 77). Running this sequence through the model will result in indexing errors

This PR adds support for truncation to the clip model. Allowing to pass in the standard sentence transformer max_seq_length argument and defaulting it to processor.tokenizer.model_max_length (i.e. 77).