fix: FM, base_distribution.sample has no argument 'seed'

[vpratz]

The recent change to use the base distribution instead of a normal distribution in FlowMatching introduced an error, see #302 for an example. This PR fixes this by removing the seed kwarg and ensuring that self.base_distribution is built.

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Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bayesflow/networks/flow_matching/flow_matching.py	0.00%	5 Missing ⚠️

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Files with missing lines	Coverage Δ
bayesflow/networks/flow_matching/flow_matching.py	82.07% <0.00%> (-5.81%)	⬇️

... and 23 files with indirect coverage changes

[LarsKue]

Looks like we should check self.built and call self.build() instead of checking self.base_distribution.

[vpratz]

Thanks for taking a look, I have adapted the code accordingly.