Make loading of pretrained gpt2 faster by avoiding initialization of Conv1D's weights
Author: twaka
Pull request: yes

# What does this PR do?
Currently, pytorch_util's Conv1D always computes `normal_` to initialize weights regardless of `init_empty_weights`.
It makes model loading time of gpt2 longer.
This PR fixes this by reordering the initialization of Conv1D to apply `normal_` after assigning weight as `nn.Parameter` to avoid unnecessary initialization computation.

gpt2-xl.py
```py
from transformers import AutoModelForCausalLM
import torch

model = AutoModelForCausalLM.from_pretrained(
        "gpt2-xl",
        torch_dtype=torch.half,
        low_cpu_mem_usage=True)
```

before
```
$ python -m cProfile -s tottime gpt2-xl.py | head
         1815860 function calls (1739023 primitive calls) in 25.421 seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
      243   19.131    0.079   19.131    0.079 {method 'normal_' of 'torch._C._TensorBase' objects}
     1256    1.820    0.001    1.820    0.001 {method '_set_from_file' of 'torch._C.StorageBase' objects}
     3045    1.427    0.000    1.427    0.000 {method 'to' of 'torch._C._TensorBase' objects}
        2    0.677    0.339    0.677    0.339 {method 'do_handshake' of '_ssl._SSLSocket' objects}
        2    0.380    0.190    0.380    0.190 {method 'read' of '_ssl._SSLSocket' objects}
```

after
```
$ python -m cProfile -s tottime gpt2-xl.py | head
         1816052 function calls (1739215 primitive calls) in 5.691 seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     1256    1.791    0.001    1.791    0.001 {method '_set_from_file' of 'torch._C.StorageBase' objects}
     3045    0.892    0.000    0.892    0.000 {method 'to' of 'torch._C._TensorBase' objects}
        2    0.676    0.338    0.676    0.338 {method 'do_handshake' of '_ssl._SSLSocket' objects}
        2    0.402    0.201    0.402    0.201 {method 'connect' of '_socket.socket' objects}
        2    0.373    0.186    0.373    0.186 {method 'read' of '_ssl._SSLSocket' objects}
```

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Fixes #21863

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