ns for fx: remove restriction on nodes with no args and only kwargs

Summary:

Removes the restriction from NS for FX on handling nodes which have
no positional arguments, such as `F.linear(input=x, weight=w, bias=b).

In order to achieve this, we delete all places in the code which
were doing things like

```
node.args[0]
```

And replace them with

```
_get_normalized_nth_input(node, gm, 0)
```

The `_get_normalized_nth_input` function is a best effort way to
get the n'th normalized input.

This is needed because some FX tools output nodes normalized to
be kwargs only, and we need to be able to handle this in NS.

Test plan:

```
python test/test_quantization.py -k test_linear_kwargs_shadow
```

Pull Request resolved: pytorch#78181

Approved by: https://github.com/z-a-f, https://github.com/hx89

test/quantization/fx/test_numeric_suite_fx.py

@@ -2034,6 +2034,34 @@ def forward(self, x):
             m, (torch.randn(1, 1, 4, 4),),
             results_len=0)
 
+    def test_linear_kwargs_shadow(self):
+
+        class M(nn.Module):
+            def __init__(self):
+                super().__init__()
+                self.w1 = nn.Parameter(torch.empty(4, 4))
+                self.b1 = nn.Parameter(torch.zeros(4))
+                torch.nn.init.kaiming_uniform_(self.w1, a=math.sqrt(5))
+
+            def forward(self, x):
+                x = F.linear(input=x, weight=self.w1, bias=self.b1)
+                return x
+
+        # note: FX graph mode quantization does not have good support
+        # for kwargs-only right now, so we pass in two unquantized
+        # models
+        m = M().eval()
+        mt = torch.fx.symbolic_trace(m)
+        mt_copy = copy.deepcopy(mt)
+
+        mt_shadows_mt_copy = add_shadow_loggers(
+            'a', mt, 'b', mt_copy, OutputLogger)
+
+        mt_shadows_mt_copy(torch.randn(4, 4))
+        act_compare_dict = extract_shadow_logger_info(
+            mt_shadows_mt_copy, OutputLogger, 'b')
+        self.assertTrue(len(act_compare_dict) == 1)
+
 
 class TestFXNumericSuiteCoreAPIsModels(FXNumericSuiteQuantizationTestCase):
     """