Populate the torch._decomp table on import (#78476)
#78041 broke TorchInductor, because of:
```
>>> from torch import _decomp
>>> import torch
>>> _decomp.get_decompositions([torch.ops.aten.leaky_relu])
{}
>>> import torch._refs.nn.functional
>>> _decomp.get_decompositions([torch.ops.aten.leaky_relu])
{<OpOverload(op='aten.leaky_relu', overload='default')>: <function leaky_relu at 0x7f5a39b56c10>, <OpOverload(op='aten.leaky_relu', overload='out')>: <function leaky_relu at 0x7f5a39b56c10>}
```
cc @Chillee
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78476
Approved by: https://github.com/Chillee
