Add --jobs 0 for git submodule update (#61311)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61311
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61152
Some related docs about `submodule.fetchJobs`
https://git-scm.com/docs/git-config#Documentation/git-config.txt-submodulefetchJobs
```
time git submodule update --init --recursive
________________________________________________________
Executed in 243.20 secs fish external
usr time 49.64 secs 213.00 micros 49.64 secs
sys time 29.27 secs 795.00 micros 29.27 secs
```
```
time git submodule update --init --recursive --jobs 4
________________________________________________________
Executed in 143.04 secs fish external
usr time 51.06 secs 246.00 micros 51.06 secs
sys time 30.96 secs 742.00 micros 30.96 secs
```
```
time git submodule update --init --recursive --jobs 8
________________________________________________________
Executed in 124.64 secs fish external
usr time 51.76 secs 264.00 micros 51.76 secs
sys time 30.49 secs 739.00 micros 30.49 secs
```
```
time git submodule update --init --recursive --jobs 0 # use all online cpus
________________________________________________________
Executed in 129.75 secs fish external
usr time 51.64 secs 181.00 micros 51.64 secs
sys time 31.49 secs 781.00 micros 31.49 secs
```
Test Plan: Imported from OSS
Reviewed By: 1ntEgr8
Differential Revision: D29560875
Pulled By: zhouzhuojie
fbshipit-source-id: 556027dffe744c66428075a8a1bf64683930aaaf
