Fix plot_lm handling of multidimensional data with y_model (#1992)

[uzairgheewala]

Description

Fix plot_lm functionality when using multidimensional data with y_model parameter. Previously, attempting to use plot_lm with both multidimensional y and y_model would result in dimension mismatch errors.
Also adds a test to check for the proper functioning of plot_lm when using multidimensional data with y_model parameter.
Addresses issue #1992.

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

Following problematic cases in this notebook now work: https://gist.github.com/colehaus/b2aa195687f53ef7c86eec2ebb3798c2

  # Works now with multidimensional data
  data = az.from_dict(
      observed_data = { "y": np.random.normal(size=(5, 7)) },
      posterior = {"y_model": np.random.randn(4, 1000, 5, 7)},
      dims={"y": ["dim1", "dim2"]},
      coords={"dim1": range(5), "dim2": range(7)}
  )
  # Both modes now work
  az.plot_lm(idata=data, y="y", plot_dim="dim1", y_model="y_model", kind_model="lines")
  az.plot_lm(idata=data, y="y", plot_dim="dim1", y_model="y_model", kind_model="hdi")

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📚 Documentation preview 📚: https://arviz--2408.org.readthedocs.build/en/2408/