input: Write a function `num_unique_paths` to find the number of unique paths from a given source to a destination in an `m x n` grid. The function should take two integer parameters, `m` and `n`, representing the number of rows and columns respectively. The function should return an integer representing the number of unique paths.
  
  assert num_unique_paths(3, 2) == 3
  assert num_unique_paths(7, 3) == 28
  assert num_unique_paths(3, 3) == 6
  assert num_unique_paths(4, 4) == 20
  assert num_unique_paths(1, 1) == 1
output: def num_unique_paths(m, n):
      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]