fix UserWarning for floor division in position encoder

[jveitchmichaelis]

Torch currently throws a UserWarning for a division op in the position encoding module.

UserWarning: __floordiv__ is deprecated, and its behavior will change in a future version of pytorch. It currently rounds toward 0 (like the 'trunc' function NOT 'floor'). 
This results in incorrect rounding for negative values. To keep the current behavior, use torch.div(a, b, rounding_mode='trunc'), or for actual floor division, use torch.div(a, b, rounding_mode='floor')

Using the trunc rounding mode retains current behaviour, but presumably floor would also be fine here as dim_t should never be negative.

EDIT: (There may be more places that I haven't triggered...!)