Toph wants to tile a rectangular $m\times n$ square grid with the $6$ types of tiles in the picture (moving the tiles is allowed, but rotating and reflecting is not). For which pairs $(m,n)$ is this possible?
1. **Assigning Numbers to the Grid:**
   We assign to each cell \((j, k)\) in the \(m \times n\) grid the number \(a_{jk} = i^{j+k}\), where \(i\) is the imaginary unit such that \(i^2 = -1\). This means that the numbers assigned to the cells will cycle through \(1, i, -1, -i\).

2. **Sum of Numbers in Each Tile:**
   We observe that the sum of the numbers in each of the 6 types of tiles is 0. This can be verified by calculating the sum for each tile configuration. For example, consider a tile covering cells \((j, k)\), \((j, k+1)\), \((j+1, k)\), and \((j+1, k+1)\):
   \[
   i^{j+k} + i^{j+k+1} + i^{j+1+k} + i^{j+1+k+1} = i^{j+k} + i^{j+k+1} + i^{j+k+1}i + i^{j+k+2} = i^{j+k}(1 + i + i + (-1)) = 0
   \]
   Similar calculations can be done for the other tile configurations.

3. **Sum of All Numbers on the Grid:**
   The sum of all numbers on the grid is given by:
   \[
   \sum_{j=0}^{m-1} \sum_{k=0}^{n-1} i^{j+k}
   \]
   This can be factored as:
   \[
   \left( \sum_{j=0}^{m-1} i^j \right) \left( \sum_{k=0}^{n-1} i^k \right)
   \]
   We need this product to be zero for the grid to be tiled. The sum \(\sum_{j=0}^{m-1} i^j\) is zero if and only if \(m\) is a multiple of 4, and similarly, \(\sum_{k=0}^{n-1} i^k\) is zero if and only if \(n\) is a multiple of 4. Therefore, for the product to be zero, either \(m\) or \(n\) must be a multiple of 4.

4. **Exclusion of \(m, n = 1\):**
   Clearly, a \(4 \times 1\) grid cannot be tiled with the given tiles, as the tiles are larger than the grid. Similarly, a \(1 \times 4\) grid cannot be tiled. Therefore, \(m\) and \(n\) cannot be 1.

5. **Representation of Integers Greater Than or Equal to 2:**
   Any integer greater than or equal to 2 can be represented in the form \(2x + 3y\) for some non-negative integers \(x\) and \(y\). This is because the smallest tile dimension is 2, and the next smallest is 3, allowing us to cover any larger dimension by combining these tiles.

6. **Conclusion:**
   Therefore, the grid can be tiled if and only if \(4 \mid m\) or \(4 \mid n\), and \(m, n \neq 1\).

The final answer is \( \boxed{ (m, n) } \) such that \(4 \mid m\) or \(4 \mid n\), and \(m, n \neq 1\).