Fix of broadcast serialization bug

[arrjon]

Simple fix for #529. However, I am not sure, if this is a fix for a symptom only or if something in the serialization has to be fixed.

[stefanradev93]

Reminds me of the "keras doesn't like lists" issues we had @LarsKue .

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bayesflow/adapters/transforms/broadcast.py	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
bayesflow/adapters/transforms/broadcast.py	82.97% <0.00%> (ø)

... and 3 files with indirect coverage changes

[vpratz]

Thanks for the fix! Maybe you use isinstance(self.expand, Sequence) or isinstance(self.expand, (list, tuple)) instead of the or statement, but apart from that, it looks good to me.