DDP training can not accept subnet address in IPV6
Labels: oncall: distributed, triaged

### 🐛 Describe the bug

When I pass a normal ipv6 address[fe80::4315:8136:2e6:13f8] to torch.distributed.run, starting command is shown below:
```shell
python -m torch.distributed.launch --rdzv_backend=static --nnodes=1 --nproc_per_node=4 --rdzv_endpoint=[fe80::121b:54ff:fe0f:41d3]:29500 testipv6.py
``` 
It can not be connected even I can ping this address successfully. Fisrtly, I thought IPV6 address is not supported in elastic launch.  But when I pass [::1] to the script, the address is connected and the training started and finished. Thus, I turned on the debug log by:
```shell
export TORCH_CPP_LOG_LEVEL=INFO
export TORCH_DISTRIBUTED_DEBUG=DETAIL
``` 
The it gave me the error like below:
```
[I socket.cpp:454] [c10d - debug] The server socket will attempt to listen on an IPv6 address.
[I socket.cpp:504] [c10d - debug] The server socket is attempting to listen on [::]:29500.
[I socket.cpp:578] [c10d] The server socket has started to listen on [::]:29500.
[I TCPStore.cpp:252] [c10d - debug] The server has started on port = 29500.
[I socket.cpp:691] [c10d - debug] The client socket will attempt to connect to an IPv6 address of (fe80::4315:8136:2e6:13f8, 29500).
[I socket.cpp:763] [c10d - trace] The client socket is attempting to connect to [xxx]:29500.
[W socket.cpp:665] [c10d] The client socket has failed to connect to [xxx]:29500 (errno: 22 - Invalid argument).
[I socket.cpp:700] [c10d - debug] The client socket will attempt to connect to an IPv4 address of (fe80::4315:8136:2e6:13f8, 29500).
[W socket.cpp:665] [c10d] The IPv4 network addresses of (fe80::4315:8136:2e6:13f8, 29500) cannot be retrieved (gai error: -9 - Address family for hostname not supported).
[W socket.cpp:665] [c10d] The IPv4 network addresses of (fe80::4315:8136:2e6:13f8, 29500) cannot be retrieved (gai error: -9 - Address family for hostname not supported).
[W socket.cpp:665] [c10d] The IPv4 network addresses of (fe80::4315:8136:2e6:13f8, 29500) cannot be retrieved (gai error: -9 - Address family for hostname not supported).
[W socket.cpp:665] [c10d] The IPv4 network addresses of (fe80::4315:8136:2e6:13f8, 29500) cannot be retrieved (gai error: -9 - Address family for hostname not supported).
[W socket.cpp:665] [c10d] The IPv4 network addresses of (fe80::4315:8136:2e6:13f8, 29500) cannot be retrieved (gai error: -9 - Address family for hostname not supported).
``` 
From the err log shown above, I can not understand why the ipv6 address is an Invalid argument. Thus I found the lastest code right at the line which caused the error. The lateset code in socket.cpp is shown below:
```c++
SocketConnectOp::ConnectResult SocketConnectOp::tryConnectCore(
    const ::addrinfo& addr) {
  int r = ::connect(socket_->handle(), addr.ai_addr, addr.ai_addrlen);
``` 
Then, I found that the error is caused by the function connect. To debug easily, I extract this code into a cpp file, to find out why the address I passed is an invalid argument. The test .cpp script is shown below:
```c++
#include <iostream>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <cstring>
#include <net/if.h>
int main() {
    int sockfd = socket(AF_INET6, SOCK_STREAM, 0);
    if (sockfd == -1) {
        std::cerr << "Socket creation failed." << std::endl;
        return 1;
    }
    sockaddr_in6 serverAddr;
    serverAddr.sin6_family = AF_INET6;
    serverAddr.sin6_port = htons(29499); // Port number
    //serverAddr.sin6_scope_id = if_nametoindex("eno1");
    if (inet_pton(AF_INET6, "fe80::4315:8136:2e6:13f8", &serverAddr.sin6_addr) <= 0) {
        std::cout << "Invalid IPv6 address." << std::endl;
        return 1;
    }

    if (connect(sockfd, (struct sockaddr*)&serverAddr, sizeof(serverAddr)) == -1) {
        perror("Connect failed");
        return 1;
    }
    std::cout << "Connected to the server!" << std::endl;
    // Close the socket
    close(sockfd);
    return 0;
}

``` 
In this script, I tried a different ipv6 address "2001:0db8:85a3:0000:0000:8a2e:0370:7334", this time, it gave me a different new error
```
Connect failed: Network is unreachable
```
Suddenly I realized that the IP address I passed may have something special that caused this error. Then, I found the IPV6 address is a subnet address started with fe80. The I tried other ip addresses not started with fe80, the invalid argument error disappeared, the new error is "network is unreachable". I think the current code can not support a subnet ipv6 address. Therefore, I add this line to the code:
```c++
serverAddr.sin6_scope_id = if_nametoindex("eno1");
``` 
Run the .cpp script again, the result is shown as below:
```
Connect failed: Connection refused
```
At this step, I realized that I passed the verfication step, and the socket tried to connect the ip but been refused. Thus, I changed the origin code in torch in socket.cpp like this:
![image](https://github.com/pytorch/pytorch/assets/55902565/85716358-6b21-4083-8594-39627fe62fe8)
Then, I compile the code, and run the elastic launch command again. This time, my training script started run and finished. Thus, I concluded that the source code does not support a subnet ipv6 address


### Versions

[pip3] flake8==6.0.0
[pip3] flake8-bugbear==23.3.23
[pip3] flake8-comprehensions==3.11.1
[pip3] flake8-executable==2.1.3
[pip3] flake8-logging-format==0.9.0
[pip3] flake8-pyi==23.3.1
[pip3] flake8-simplify==0.19.3
[pip3] mypy==0.960
[pip3] mypy-extensions==1.0.0
[pip3] numpy==1.23.1
[pip3] torch==2.1.0.dev20230810+cpu
[pip3] torchvision==0.15.1
[conda] numpy 1.23.1 pypi_0 pypi
[conda] torchvision 0.15.1 pypi_0 pypi

cc @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @osalpekar @jiayisuse @H-Huang @kwen2501 @awgu @penguinwu