Bug in accelerator data transfer cost

[sjoks93]

line 399 in accelerator.py:

    nb_receiver_memory_writes_for_data = ceil(
        tensor.size / sender_top_memory_level.write_bw
    )

should be:

    nb_receiver_memory_writes_for_data = ceil(
        tensor.size / receiver_top_memory_level.write_bw
    )