input: Write a function to find the number of unique paths from a given source to a destination in an m x n grid. The function should return the total number of unique paths from the top-left corner to the bottom-right corner, assuming only right and down moves are allowed. If the grid is 1x1, it should return 1.
  assert num_unique_paths(3, 3)==6
  assert num_unique_paths(2, 2)==2
  assert num_unique_paths(1, 1)==1
  assert num_unique_paths(3, 2)==3
  assert num_unique_paths(7, 3)==28
output: def num_unique_paths(m, n):
      if m == 1 or n == 1:
          return 1
      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]