Add NVFuser Python Frontend
New functionality.
1. Adds Pybind11 bindings for NVFuser.
2. Requires a build file change and JIT python file change outside of NVFuser's code area.
Example:
```
import torch
from torch._C._nvfuser import Fusion, FusionDefinition
# Construct and Define Fusion
fusion = Fusion()
with FusionDefinition(fusion) as fd :
t0 = fd.define_tensor(3)
t1 = fd.define_tensor(1)
s0 = fd.define_scalar()
fd.add_input(t0)
fd.add_input(t1)
fd.add_input(s0)
c0 = fd.define_constant(3.0)
t1_b = fd.Ops.broadcast(t1, [True, True, False])
t2 = fd.Ops.add(t0, t1)
t3 = fd.Ops.mul(t2, c0)
t4 = fd.Ops.mul(t3, s0)
t5 = fd.Ops.relu(t4)
t6 = fd.Ops.sum(t5, [-1], False)
fd.add_output(t6)
fusion.print_ir()
# Execute Fusion
input1 = torch.ones(2, 4, 8, device='cuda')
input2 = torch.ones(8, device='cuda')
# Kernel compilation should be cached for the 2nd iteration
# with input tensors of the same shape
for _ in range(5) :
outputs = fusion.execute([input1, input2, 2.0])
print(outputs[0])
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76353
Approved by: https://github.com/csarofeen, https://github.com/mruberry
