Implement delete for InMemoryVectorStore + throw exception if not implemented

[raffaem]

Checked

• I searched existing ideas and did not find a similar one
• I added a very descriptive title
• I've clearly described the feature request and motivation for it

Feature request

I think that:

1. If the delete method of a VectorStore is not implemented, it should throw an exception rather than returning None
2. The method delete for InMemoryVectorStore should be implemented

Motivation

1. I was running my RAG on old documents because I didn't realize the method was unimplemented. The user can catch the exception anyway. I think the standard is to throw an exception in these cases
2. I think implementing the method for the in-memory vector store should not be too difficult

Proposal (If applicable)

No response