hi ! i do not know if this is considered sleep training but at abt 1-2 months plus, it became a habit whereby my baby would sleep on his tummy , on my chest throughout the night. I then realised that was the best way for both my baby and i to get a good nights sleep.
When he was going 3 months, i transitioned and made him sleep on his own and thankfully he did. So it became a routine for him to sleep through the night even when its already not on me.
I do not know if my way is recommended but this was how my son got used to sleep through the nigh. Hope this helps.