Collections: set batch size default to 10

[nishika26]

Describe the current behavior
The batch_size parameter in create collection api controls how many documents are sent to OpenAI in a
single transaction when creating the vector store. our default value for this parameter was 1 which would cause a bigger set of files to take long time to upload, hence causing time limit error from celery's side.

Describe the enhancement you'd like
If not specified, the default value will be 10 now . This helps optimize the upload process for large document sets because now 10 files will be uploaded to vector store in a batch, and not one file at a time