`download=True` argument for `VOCDetection` will always download and rewrite the dataset again

[tttyuntian]

🐛 Describe the bug

The command for loading VOCDetection will always take a long time if download=True is set. I am using torchvision version of 0.22.0. I'm unsure if it redownloads the zip file every time, or it always tries to unzip the file again and again.

python
voc = torchvision.datasets.VOCDetection(root=root, image_set="val", download=True)  # Always take ~8min
voc = torchvision.datasets.VOCDetection(root=root, image_set="val")  # Take ~0.0s

Versions

Collecting environment information...
PyTorch version: 2.7.0+cu126
Is debug build: False
CUDA used to build PyTorch: 12.6
ROCM used to build PyTorch: N/A

OS: Red Hat Enterprise Linux 9.2 (Plow) (x86_64)
GCC version: (GCC) 11.3.1 20221121 (Red Hat 11.3.1-4)
Clang version: Could not collect
CMake version: version 3.20.2
Libc version: glibc-2.34

Python version: 3.9.16 (main, Dec  8 2022, 00:00:00)  [GCC 11.3.1 20221121 (Red Hat 11.3.1-4)] (64-bit runtime)
Python platform: Linux-5.14.0-284.11.1.el9_2_cifs.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 GeForce RTX 3090
Nvidia driver version: 535.129.03
cuDNN version: Could not collect
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

CPU:
Architecture:                    x86_64
CPU op-mode(s):                  32-bit, 64-bit
Address sizes:                   43 bits physical, 48 bits virtual
Byte Order:                      Little Endian
CPU(s):                          64
On-line CPU(s) list:             0-63
Vendor ID:                       AuthenticAMD
Model name:                      AMD EPYC 7532 32-Core Processor
CPU family:                      23
Model:                           49
Thread(s) per core:              1
Core(s) per socket:              32
Socket(s):                       2
Stepping:                        0
Frequency boost:                 enabled
CPU max MHz:                     2400.0000
CPU min MHz:                     1500.0000
BogoMIPS:                        4800.20
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 nopl nonstop_tsc cpuid extd_apicid aperfmperf rapl pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 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 ibrs ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 cqm rdt_a rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local clzero irperf xsaveerptr rdpru wbnoinvd amd_ppin arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif v_spec_ctrl umip rdpid overflow_recov succor smca sme sev sev_es
Virtualization:                  AMD-V
L1d cache:                       2 MiB (64 instances)
L1i cache:                       2 MiB (64 instances)
L2 cache:                        32 MiB (64 instances)
L3 cache:                        512 MiB (32 instances)
NUMA node(s):                    2
NUMA node0 CPU(s):               0-31
NUMA node1 CPU(s):               32-63
Vulnerability Itlb multihit:     Not affected
Vulnerability L1tf:              Not affected
Vulnerability Mds:               Not affected
Vulnerability Meltdown:          Not affected
Vulnerability Mmio stale data:   Not affected
Vulnerability Retbleed:          Mitigation; untrained return thunk; SMT disabled
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; Retpolines, IBPB conditional, STIBP disabled, RSB filling, PBRSB-eIBRS Not affected
Vulnerability Srbds:             Not affected
Vulnerability Tsx async abort:   Not affected

Versions of relevant libraries:
[pip3] numpy==2.0.2
[pip3] nvidia-cublas-cu12==12.6.4.1
[pip3] nvidia-cuda-cupti-cu12==12.6.80
[pip3] nvidia-cuda-nvrtc-cu12==12.6.77
[pip3] nvidia-cuda-runtime-cu12==12.6.77
[pip3] nvidia-cudnn-cu12==9.5.1.17
[pip3] nvidia-cufft-cu12==11.3.0.4
[pip3] nvidia-curand-cu12==10.3.7.77
[pip3] nvidia-cusolver-cu12==11.7.1.2
[pip3] nvidia-cusparse-cu12==12.5.4.2
[pip3] nvidia-cusparselt-cu12==0.6.3
[pip3] nvidia-nccl-cu12==2.26.2
[pip3] nvidia-nvjitlink-cu12==12.6.85
[pip3] nvidia-nvtx-cu12==12.6.77
[pip3] torch==2.7.0
[pip3] torchvision==0.22.0
[pip3] triton==3.3.0
[conda] Could not collect

[NicolasHug]

Yeah, it gets re-downloaded if download is set to True. I think the reason for this behavior is that this ensures the data is always correct and from the source, and cannot have been tempered-with. I agree it doesn't lead to the best UX though.

I think we've changed this behavior in some of the datasets already, we could align the rest