Add option for projecting the `TransformerBlock` output + specifying the activation function `tf_out_act`

[sararb]

• Add the option to project the output of the TransformerBlock pos_emb_pred with specific activation function tf_out_act:

pos_emb_pred =tf_out_act(nn.Linear(d_model, transformer_output_projection_dim)(pos_emb_pred)

• This option is needed when the user want to project the output of the Transformer to specific dimension. For example to item_id embedding when using weight tying.

• This projection will be included in the body of SequentialPredictionTask.

• The projection_flag, the tf_out_act and the transformer_output_projection_dim should be set in the init of SequentialPredictionTask