GH-1089: Optimize FlairEmbeddings depending on storage mode

[alanakbik]

As discussed in #1089, the clone() operation in FlairEmbeddings is expensive and only required if the embedding_storage_mode is 'gpu'. However, information of which storage mode is selected is not visible in FlairEmbeddings. To address this, this PR adds a new global variable embedding_storage_mode that is set in the store_embeddings() method and checked in FlairEmbeddings. This gives us a roughly 10% improvement in inference speed.

The PR also renames the embeddings_storage_mode parameter of the evaluate() functions to embedding_storage_mode to make it consistent.

[MichaelHintz]

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[adizdari]

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