not create new symbol with immutable shape convolution.

[fukatani]

Resolve #1763
If conv uses custom_pad and pad[r] - effective_ks[r] == -1 and strides[r] == 1, output_shape equals input_shape.
In this case we should not create new symbol for shape.

I confirmed flexible MultiHeadAttention works with this change.

import torch  # 1.13.1
import numpy as np  # 1.22.3
import coremltools as ct  # 6.2

from ane_transformers.reference.multihead_attention import MultiHeadAttention

N = 10
x = torch.rand(1, 512, 1, N)
y = torch.rand(1, 512, 1, N)
z = torch.rand(1, 512, 1, N)

with torch.no_grad():
    layer = MultiHeadAttention(512, n_head=8, dropout=0.0).eval()
    jit = torch.jit.trace(layer, (x, y, z))

    # Flexible input shape - fails
    flexible_shape = ct.Shape(shape=(1, 512, 1, ct.RangeDim(1, 448)))
    mlmod_flexible_shape = ct.convert(
        jit,
        inputs=[
            ct.TensorType("q", flexible_shape),
            ct.TensorType("k", flexible_shape),
            ct.TensorType("v", flexible_shape),
        ]
    )

    out = layer(x, x, x)
    out_dict = mlmod_flexible_shape.predict({'q': x.detach().numpy().astype(np.float32),
                                             'k': x.detach().numpy().astype(np.float32),
                                             'v': x.detach().numpy().astype(np.float32)})
    np.allclose(out[0], out_dict['var_195'], rtol=0.001, atol=0.001)  # OK

[YifanShenSZ]

Looks perfect, thanks for the fix!

Kicked off CI, will approve once it turns green.

[YifanShenSZ]

CI passed, approved, thanks again!