Ok running with 1.3 on linux I can repro.
Also I am very worried as I reduced your example even further to:
import torch
import torch.nn as nn

# <= 1000 works
# > 1000 does not work
val = 1001

mod = torch.nn.Embedding(100, 64, max_norm=1.0)
v0 = torch.rand(val).mul(100).long()

out = mod(v0)
print(out.grad_fn)
print(out.grad_fn.next_functions) # This should show an AccumulateGrad. If it's a CopySlices, it will fail below
print(out.sum().backward())

Could you check what is the breakpoint value for you? Is it the same?