[iOS] podspec file for Cocoapods release, pod name: LibTorchvision (#4055) (#4074)

Co-authored-by: Yuchen Huang <hyc@fb.com>
Co-authored-by: Francisco Massa <fvsmassa@gmail.com>

ios/LibTorchvision.podspec

+pytorch_version = '1.9.0'
+
+Pod::Spec.new do |s|
+    s.name             = 'LibTorchvision'
+    s.version          = '0.10.0'
+    s.authors          = 'PyTorch Team'
+    s.license          = { :type => 'BSD' }
+    s.homepage         = 'https://github.com/pytorch/vision'
+    s.source           = { :http => "https://ossci-ios.s3.amazonaws.com/libtorchvision_ops_ios_#{s.version}.zip" }
+    s.summary          = '"The C++ library of TorchVision ops for iOS'
+    s.description      = <<-DESC
+        The C++ library of TorchVision ops for iOS.
+        This version (#{s.version}) requires the installation of LibTorch #{pytorch_version} or LibTorch-Lite #{pytorch_version}.
+    DESC
+    s.ios.deployment_target = '12.0'
+    s.vendored_libraries = 'install/lib/*.a'
+    s.user_target_xcconfig = {
+        'VALID_ARCHS' => 'x86_64 arm64',
+        'OTHER_LDFLAGS' => '$(inherited) -force_load "$(PODS_ROOT)/LibTorchvision/install/lib/libtorchvision_ops.a"',
+        'CLANG_CXX_LANGUAGE_STANDARD' => 'c++14',
+        'CLANG_CXX_LIBRARY' => 'libc++'
+    }
+    s.library = ['c++', 'stdc++']
+end