选择二维列表中的所有非空子矩形

def find_submatrices(matrix):
    rows = len(matrix)
    cols = len(matrix[0])
    submatrices = []

    for i in range(rows):
        for j in range(cols):
            for k in range(i, rows):
                for l in range(j, cols):
                    submatrix = []
                    for m in range(i, k+1):
                        row = []
                        for n in range(j, l+1):
                            row.append(matrix[m][n])
                        submatrix.append(row)
                    submatrices.append(submatrix)

    return submatrices