Definition: In this task, you need to reverse all words of a given length in the sentence. The number of letters in a word determine its length. For example, the length of the word "apple" is 5.
Input: Sentence: 'a young man smiling while looking at a young person displaying an obscene gesture with her fingers'. Reverse all words of length '7' in the given sentence.

a young man gnilims while gnikool at a young person displaying an enecsbo erutseg with her sregnif