input: Write a function to find the number of unique paths from a given source to a destination in an m x n grid where movement is restricted to right and down only. If either m or n is 0, then return 0.
  assert num_unique_paths(3, 2) == 3
  assert num_unique_paths(2, 3) == 3
  assert num_unique_paths(7, 3) == 28
  assert num_unique_paths(0, 3) == 0
  assert num_unique_paths(2, 0) == 0
output: def num_unique_paths(m, n):
      if m == 0 or n == 0:
          return 0
      dp = [[0] * n for _ in range(m)]
      for i in range(m):
          dp[i][0] = 1
      for j in range(n):
          dp[0][j] = 1
      for i in range(1, m):
          for j in range(1, n):
              dp[i][j] = dp[i-1][j] + dp[i][j-1]
      return dp[m-1][n-1]