[ExecuTorch] Cannot Use Dynamic Index to Select

This toy model fails in ExecuTorch
```
class M(torch.nn.Module):
def forward(self, float_arr, int_arr):
dynamic_index = int_arr[1]
float_arr[dynamic_index] = 12.95
return float_arr
```
due to
```
E torch._dynamo.exc.UserError: Consider annotating your code using torch._constrain_as_*(). Could not guard on data-dependent expression u0 (unhinted: u0). (Size-like symbols: none)
E
E Potential framework code culprit (scroll up for full backtrace):
E File ".223", line 8, in forward
E l_float_arr_[dynamic_index] = 12.95; setitem = l_float_arr_; dynamic_index = None
E
E For more information, run with TORCH_LOGS="dynamic"
E For extended logs when we create symbols, also add TORCHDYNAMO_EXTENDED_DEBUG_CREATE_SYMBOL="u0"
E If you suspect the guard was triggered from C++, add TORCHDYNAMO_EXTENDED_DEBUG_CPP=1
E For more debugging help, see https://docs.google.com/document/d/1HSuTTVvYH1pTew89Rtpeu84Ht3nQEFTYhAX3Ypa_xJs/edit?usp=sharing
E
E For C++ stack trace, run with TORCHDYNAMO_EXTENDED_DEBUG_CPP=1
E For more information about this error, see: https://pytorch.org/docs/main/generated/exportdb/index.html#constrain-as-size-example
```
But some other dynamic indexing models are fine, e.g.
```
class IndexPutModel(torch.nn.Module):
def forward(self, x, position, val):
y = x.clone()
y[:, position] = val
return y
```

Start by running the toy model in the issue and compare its dynamic indexing behavior with IndexPutModel. Trace the ExecuTorch conversion path that handles indexed assignment; done means the toy model converts and runs without the reported data-dependent-expression error.

Written by the indexing model from the issue text.