The initial plans for the invasion did not have any plans for the atomic bomb use in them, because they were made well in advance of the testing of the bombs and their properties, reliability, and so on were not reliably known.

After the bombing of Hiroshima and Nagasaki, there were the beginnings of thoughts voiced by people like General Marshall and others that perhaps any future atomic bombs ought to be saved up and reserved for use in an invasion. General Groves was asked, by Marshall, what their tactical properties might be (e.g., if you attacked an area, how long would it be before troops could walk through it?). But the war ended before any of these plans could be made more concrete — they were ideas being contemplated, but nothing was put into motion. The possibility of them being used in a tactical way during the invasion is one of many possibilities that were not resolved one way or another.

For more information, see Barton J. Bernstein, "Eclipsed by Hiroshima and Nagasaki: Early Thinking about Tactical Nuclear Weapons," International Security 15, no. 4 (Spring 1991), 149-173.