### RGB to Grayscale
**Example:** Convert an RGB image to grayscale.
**Constraints:** The input image will be a 3D numpy array.
Test Cases
Test Case 1
Input:
[[[255, 0, 0], [0, 255, 0], [0, 0, 255]]]Expected:
[[76.5], [76.5], [76.5]]Test Case 2
Input:
[[[255, 255, 255], [0, 0, 0], [128, 128, 128]]]Expected:
[[255.0], [0.0], [128.0]]+ 3 hidden test cases