[Fix] Remove double loading of data

[antonymilne]

Description

A special treat for @petar-qb to find when he's feeling better 😀 Probably the easiest optimisation we could have made. Thank you to him for spotting this horrible inefficiency that I had written in the code!

Functionally this is exactly the same as before. It's just better performing. Previously doing kwargs.setdefault always ran the .load() function. Now we only want to run it if it's strictly required (which in Vizro at the moment it actually never is, but I saw someone doing graph() the other day so they were using it, and maybe we will in future).

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[huong-li-nguyen]

LGTM 👍

[petar-qb]

What a warm welcome! Thanks for handling this A 🚀