MergeDimsLayer should use keep_order=True (and disallow keep_order=False)

[albertz]

This is to follow the principle that the order of axes should never matter.
(keep_order is about using the user specified order in the layer axes argument, and ignoring the incoming data axes order.)

This probably needs a new behavior version (#508).

[albertz]

I now realize that this will also require consistent and unique dim tags (#632) because sth like "axes": "static" would merge all static axes, but again the order is not well defined here. So sth like this would be disallowed. You would have to use dim tags and specify it like "axes": [dim_tag_1, dim_tag_2] or so.

[albertz]

I now realize that this will also require consistent and unique dim tags (#632) because sth like "axes": "static" would merge all static axes

This is already the case right now.

    if keep_order:
      assert isinstance(axes, (tuple, list)), "%s: unique axes %r required" % (self, axes)
      axes_ = []
      for axis in axes:
        axis_ = self.input_data.get_axes_from_description(axis, allow_int=False)
        assert len(axis_) <= 1, "%s: unique axes %r required, but got %r -> %r" % (self, axes, axis, axis_)
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