[BUG] Add documents validation failure when use_existing_tensors=True

[farshidz]

Describe the bug
When adding documents with use_existing_tensors=True and an invalid document that is not a dictionary, we get a 500 error instead of the expected 400 that we get use_existing_tensors=False.

To Reproduce

mq_os.create_index('my-index-1')

mq_os.index('my-index-1').add_documents(
    [
        1234
    ],
    tensor_fields=['title', 'age'],
    use_existing_tensors=True
)

Expected behavior
400

Screenshots
N/A

Desktop (please complete the following information):
Tested with macOS, but N/A

Additional context
This issue happens because validation takes place after use_existing_tensors logic assumes the document is a dictionary. See

marqo/src/marqo/tensor_search/tensor_search.py

Line 432 in 6af4c94

validation.validate_doc(doc)