Correction: memory_ops for encoder_forward

[MasterSkepticista]

Two changes:

• sizeof(floatX), otherwise bandwidth calculation is off by 2x when using bf16
• 3C memory ops instead of 4C: For each input token we have 2C reads and C writes.

On RTX-3090 (936 GB/s):

# Before
...
block_size 1024 | time 0.0483 ms | bandwidth 2082.91 GB/s

# After
...
block_size 1024 | time 0.0483 ms | bandwidth 782.40 GB/s