Thank you for your answer!
Could you elaborate a little with me and tell me If I perceived anything not correctly?
I have a dataframe with 2 columns (sentiment and text) where the text is already preprocessed.
In the init() method of the Dataset class, the dataframe is given as an argument and then I should proceed with the vectorization process.  In the vectorization process, I need to create a vocabulary of each word contained in the dataset and map it to an integer, right?  Then I will create a third column called text_vectorized, where each sentece will be a list containing integers that correspond to the words of the sentence.
In the len() method i should return the size of the whole dataset, right?
Last but not least,  In the getitem() method, an index must be given as an argument in order to retrieve the corresponding vectorized text along with its label.
Have I missed something, or am I okay to go?