```gdscript
func display_layer_data():
	# Assume TileMapLayer node is referenced as 'tile_map_layer'
	var tile_set = self.tile_set

	# Get the number of TileSetSources (sources in tileset)
	var source_count = tile_set.get_source_count()

	print("Number of sources in TileSet: ", source_count)

	# Iterate over all sources in the tileset
	for i in range(source_count):
		var source_id = tile_set.get_source_id(i)  # integer ID of source (atlas source)
		
		# Get the source resource
		var source = tile_set.get_source(source_id)
		
		# Check if this source is an Atlas type source
		if source is TileSetAtlasSource:
			print("Found Atlas Source ID: ", source_id)
			print("Atlas Grid Size: ", source.get_atlas_grid_size())
			# You can now use this source_id as the valid atlas source ID assigned to the TileMapLayer
```