Implement an RGB to grayscale conversion kernel that matches the reference implementation.
The kernel should convert square RGB images with even sizes to grayscale using the standard coefficients:
Y = 0.2989 R + 0.5870 G + 0.1140 B

Input: RGB tensor of shape (H, W, 3) with values in [0, 1]
Output: Grayscale tensor of shape (H, W) with values in [0, 1]
