Remove unintentional "double" assignment  in TF-BART like models
This PR fixes unintentionally used "double" assignment during reshaping of `attn_wegihts` in the TF BART-like models.

**Description:** Replace `attn_weights = attn_wegihts = tf.reshape(...)` with `attn_weights = tf.reshape(...)` and thus remove unintentionally used "double" assignment.

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Reviewer: @jplu