call BatchSample method instead of repeating content

[TillHae]

Description

In get_num_source_steps - ModelBatch everything from get_num_steps - BatchSample is just repeated. Instead we can also just call the method on the parameter target_samples, which is a BacthSample object.

CC @clessig

Issue Number

Closes #1791

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[grassesi]

This does exactly what it says. I also noticed this function is not used throughout the entire codebase. Instead downstream code in the weathergen.model subpackage directly uses BatchSamples.get_num_steps. Maybe this method should be entirely removed. What do you think @clessig ?

[clessig]

The underlying problem is that the function should read:

    def get_num_source_steps(self) -> int:
        """
        Get number of input/source steps from smallest of all available streams
        """
        # TODO: define explicitly
        lens = [
            len(stream.source_tokens_cells)
            for _, stream in self.source_samples.samples[0].streams_data.items()
        ]

        return min(lens)

That is, it should be for _, stream in self.source_samples.samples[0].streams_data.items() instead of for _, stream in self.target_samples.samples[0].streams_data.items(). Even better would be to also add a function get_num_source_steps() to StreamData and perhaps to the intermediate classes like Sample.

@TillHae : could you take care of this?

[clessig]

See last text comment

[TillHae]

What I did with commit d79f737:

• added getter for source and target steps to StreamData
• added getter for source and target steps, which iterates through all streams, gets the corresponding length from the StreamData getter and returns the minimum to Sample
• removed get_num_steps from BatchSamples and added get_num_source_steps and get_num_target_steps, which both just use the Sample getter
• due to the removal of get_num_steps I needed to change all occurences to get_num_source_steps
• the getter in ModelBatch also just use the getter from BatchSamples