Fix Type Mismatch (#6410)
`num_bytes_per_thread` was a smaller type than `file_num_bytes`, this
caused issues when dividing by `num_threads`.
Co-authored-by: jomayeri <deepspeed@H100-VM2.shlnn55tgwve1eacvp21ie45dg.jx.internal.cloudapp.net>
Co-authored-by: Olatunji Ruwase <olruwase@microsoft.com>
