[iOS] Add podspec for libTorch-lite nightly build (#62691)
Summary:
The nightly pod version will be aliased with [PyTorch nightly build version](https://l.facebook.com/l.php?u=https%3A%2F%2Fgithub.com%2Fpytorch%2Fpytorch%2Fblob%2Fmaster%2F.circleci%2Fscripts%2Fbinary_populate_env.sh%23L88&h=AT3AeTpSGcz9YVeG7Lr_bweWOv8H2-kAMevglFfMslaZwgEPptNM59WdWj2ZER806rKVLNhQGM5EQcyFC_8xOq334LBo2J6YzgPW2LELkgASlA6UxP2gaD2 (https://github.com/pytorch/pytorch/commit/fa22f6303f5cf23058accf899723d6f589066ddf)Wy5mA6_lu_YlHHbEGPIU7ewJQD1 (https://github.com/pytorch/pytorch/commit/2d884f226365f94833df91de532e3a31b0db310d)aBSlOy) and [CocoaPods version specification](https://l.facebook.com/l.php?u=https%3A%2F%2Fguides.cocoapods.org%2Fusing%2Fthe-podfile.html%23specifying-pod-versions&h=AT3AeTpSGcz9YVeG7Lr_bweWOv8H2-kAMevglFfMslaZwgEPptNM59WdWj2ZER806rKVLNhQGM5EQcyFC_8xOq334LBo2J6YzgPW2LELkgASlA6UxP2gaD2 (https://github.com/pytorch/pytorch/commit/fa22f6303f5cf23058accf899723d6f589066ddf)Wy5mA6_lu_YlHHbEGPIU7ewJQD1 (https://github.com/pytorch/pytorch/commit/2d884f226365f94833df91de532e3a31b0db310d)aBSlOy), the version format of the podspect is `PyTorch version + nightly build date`, like `1.10.0.20210812`.
Usage:
1. Add `pod 'LibTorch-Lite-Nightly'` to `Podfile`
2. Run `pod install` to install the nightly built lib
3. Run `pod update` to update the lib to the latest version
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62691
Test Plan:
* Test on [TestApp](https://github.com/pytorch/pytorch/tree/master/ios/TestApp) and [HelloWorld](https://github.com/pytorch/ios-demo-app):
Podfile: `pod 'LibTorch-Lite-Nightly'`
* Test on Private Pod:
{F642106928}
Reviewed By: xta0
Differential Revision: D30090760
Pulled By: hanton
fbshipit-source-id: 361aa2ed24a11d6aced8374cb45f70f49bd5da52
