MostSimilarDocumentsPipeline doesn't have pipeline property

[vblagoje]

Describe the bug
MostSimilarDocumentsPipeline, unlike all other pipelines, doesn't have the underlying pipeline property/field. As a consequence, any method from the superclass that replies on pipeline (get_type, save_to_yaml) will fail.

Expected behavior
Just like any other pipeline e.g. SearchSummarizationPipeline, ExtractiveQAPipeline etc., the above-mentioned methods should be supported.

To Reproduce
Create MostSimilarDocumentsPipeline and invoke the above-mentioned methods.

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