You are given two $m \times n$ binary matrices grid1 and grid2 containing only $0$s representing water and $1$s representing land. An island is defined as a group of $1$s connected $4$ directionally (horizontal or vertical). All cells outside the boundary of the grid are considered water cells.

An island in grid2 is considered a sub island if every cell belonging to that island corresponds to a cell with value $1$ in grid1.

Return the total number of islands in grid2 that are sub islands.

Constraints:

• m $==$ grid1.length $==$ grid2.length

• n $==$ grid1[i].length $==$ grid2[i].length

• $1 \leq$ m, n $\leq 500$

• grid1[i][j] and grid2[i][j] are either $0$ or $1$.

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