What's the meaning of '2876934.2 / 1281167 * self.data_len' when calculating instance_Z?

Thanks for this great work.

However, there still exits a problem making me confused, that is how to calculate the Z in NCE loss.
In the implementation of the code, I found that Z is defined as '2876934.2 / 1281167 * self.data_len',
I wondering what's the meaning of 2876934.2 and 1281167

THANKS FOR ANY HELPFUL ADVICES：）
