`import functorch.dim` silently breaks `torch.Tensor.expand(size=...)`

### 🐛 Describe the bug

After `import functorch.dim`, `torch.Tensor.expand` silently ignores the `size=` keyword argument, returning the input tensor unchanged instead of expanding it.

### Root cause

[functorch/dim/\_\_init\_\_.py#106](https://github.com/pytorch/pytorch/blob/v2.8.0/functorch/dim/__init__.py#L106) replaces `torch.Tensor.expand` with a C wrapper:

```python
torch.Tensor.expand = _C._instancemethod(_C.expand)
```

The replacement changes the type of `torch.Tensor.expand` from `method_descriptor` to `builtin_function_or_method`.
The new version does not accept the `size=` keyword argument — it silently ignores it and returns the input tensor as-is, without raising any error.

### Reproducing code

```python
import torch

def test_expand(input: torch.Tensor, size: tuple[int, ...]) -> None:
output_args = input.expand(*size)
output_kwargs = input.expand(size=size)
assert output_args.shape == size, (
f"The shape of the output of `torch.Tensor.expand(self, *size)` "
f"should be `{size}` but it is different: `{output_args.shape}`."
)
assert output_kwargs.shape == size, (
f"The shape of the output of `torch.Tensor.expand(self, size=size)` "
f"should be `{size}` but it is different: `{output_kwargs.shape}`."
)

input = torch.zeros(1)
size = (10, 1)

test_expand(input, size)

print("Before importing `functorch.dim`: OK")

import functorch.dim

test_expand(input, size)

print("After importing `functorch.dim`: OK")
```

### Output

```console
Before importing `functorch.dim`: OK
Traceback (most recent call last):
File "/home/8/ue06348/projects/SSDP/test.py", line 27, in
test_expand(input, size)
File "/home/8/ue06348/projects/SSDP/test.py", line 11, in test_expand
assert output_kwargs.shape == size, (
^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: The shape of the output of `torch.Tensor.expand(self, size=size)` should be `(10, 1)` but it is different: `torch.Size([1])`.
```

### Expected behavior

`torch.Tensor.expand(size=(...))` should work the same before and after `import functorch.dim`.

### Notes

The issue also affects any library that transitively imports `functorch.dim` (e.g. `import tensordict` triggers it).

### Versions

```console
$ curl -sL https://raw.githubusercontent.com/pytorch/pytorch/main/torch/utils/collect_env.py | uv run python
Collecting environment information...
PyTorch version: 2.8.0+cu129
Is debug build: False
CUDA used to build PyTorch: 12.9
ROCM used to build PyTorch: N/A

OS: Red Hat Enterprise Linux 9.4 (Plow) (x86_64)
GCC version: (GCC) 11.4.1 20231218 (Red Hat 11.4.1-3)
Clang version: 17.0.6 (Red Hat, Inc. 17.0.6-5.el9)
CMake version: version 3.26.5
Libc version: glibc-2.34

Python version: 3.11.13 (main, Sep 18 2025, 19:46:39) [Clang 20.1.4 ] (64-bit runtime)
Python platform: Linux-5.14.0-503.40.1.el9_5.x86_64-x86_64-with-glibc2.34
Is CUDA available: True
CUDA runtime version: Could not collect
CUDA_MODULE_LOADING set to: LAZY
GPU models and configuration:
GPU 0: NVIDIA H100
MIG 4g.47gb Device 0:

Nvidia driver version: 575.57.08
cuDNN version: Could not collect
Is XPU available: False
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True
Caching allocator config: N/A

CPU:
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Address sizes: 52 bits physical, 57 bits virtual
Byte Order: Little Endian
CPU(s): 384
On-line CPU(s) list: 0-383
Vendor ID: AuthenticAMD
Model name: AMD EPYC 9654 96-Core Processor
CPU family: 25
Model: 17
Thread(s) per core: 2
Core(s) per socket: 96
Socket(s): 2
Stepping: 1
Frequency boost: enabled
CPU(s) scaling MHz: 68%
CPU max MHz: 3707.8120
CPU min MHz: 1500.0000
BogoMIPS: 4792.45
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good amd_lbr_v2 nopl nonstop_tsc cpuid extd_apicid aperfmperf rapl pni pclmulqdq monitor ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb cat_l3 cdp_l3 hw_pstate ssbd mba perfmon_v2 ibrs ibpb stibp ibrs_enhanced vmmcall fsgsbase bmi1 avx2 smep bmi2 erms invpcid cqm rdt_a avx512f avx512dq rdseed adx smap avx512ifma clflushopt clwb avx512cd sha_ni avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local avx512_bf16 clzero irperf xsaveerptr rdpru wbnoinvd amd_ppin cppc arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif x2avic v_spec_ctrl vnmi avx512vbmi umip pku ospke avx512_vbmi2 gfni vaes vpclmulqdq avx512_vnni avx512_bitalg avx512_vpopcntdq la57 rdpid overflow_recov succor smca fsrm flush_l1d debug_swap
Virtualization: AMD-V
L1d cache: 6 MiB (192 instances)
L1i cache: 6 MiB (192 instances)
L2 cache: 192 MiB (192 instances)
L3 cache: 768 MiB (24 instances)
NUMA node(s): 2
NUMA node0 CPU(s): 0-95,192-287
NUMA node1 CPU(s): 96-191,288-383
Vulnerability Gather data sampling: Not affected
Vulnerability Itlb multihit: Not affected
Vulnerability L1tf: Not affected
Vulnerability Mds: Not affected
Vulnerability Meltdown: Not affected
Vulnerability Mmio stale data: Not affected
Vulnerability Reg file data sampling: Not affected
Vulnerability Retbleed: Not affected
Vulnerability Spec rstack overflow: Mitigation; Safe RET
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2: Mitigation; Enhanced / Automatic IBRS; IBPB conditional; STIBP always-on; RSB filling; PBRSB-eIBRS Not affected; BHI Not affected
Vulnerability Srbds: Not affected
Vulnerability Tsx async abort: Not affected

Versions of relevant libraries:
[pip3] Could not collect
[conda] Could not collect
```

cc @Chillee @samdow @kshitij12345