Replace strided slice with tf.expand_dims
# What does this PR do?

This PR aims to replace the strided slice notation by its TF operator counterpart. As proposed by @mfuntowicz https://github.com/huggingface/transformers/pull/9890#discussion_r571939682
> When two new dimensions are need, can we use a different operator to make the code more readable? Suggested tf.reshape but there might be something else available?

Yes, this is doable with `tf.reshape`.<|||||>All the slow tests of the concerned models are ok!