Gradle tasks for publishing to bintray, jcenter, mavencentral etc. (#25351)

Summary:
Gradle tasks for publishing to bintray and jcenter, mavencentral; snapshot buidls go to oss.sonatype.org

Those gradle changes adds tasks:

bintrayUpload - publishing on bintray, in 'facebook' org
uploadArchives - uploading to maven repos

Gradle tasks are copied from facebook open sourced libraries like https://github.com/facebook/litho, https://github.com/facebookincubator/spectrum

To do the publishing we need to provide somehow (e.g. in ~/.gradle/gradle.properties)
```
signing.keyId=
signing.password=
signing.secretKeyRingFile=

bintrayUsername=
bintrayApiKey=
bintrayGpgPassword=

SONATYPE_NEXUS_USERNAME=
SONATYPE_NEXUS_PASSWORD=
```

android/libs/fbjni is submodule, to be able to add publishing tasks to it (it needs to be published as separate maven dependency) - I created `android/libs/fbjni_local` that has only `build.gradle` with release tasks.

pytorch_android dependency for ':fbjni' changed from implementation -> api as implementation treated as 'private' dependency which is translated to scope=runtime in maven pom file, api works as 'compile'

Testing:
it's already published on bintray with version 0.0.4 and can be used in gradle files as

```
repositories {
    maven {
        url  "https://dl.bintray.com/facebook/maven"
    }
}

dependencies {
    implementation 'com.facebook:pytorch_android:0.0.4'
    implementation 'com.facebook:pytorch_android_torchvision:0.0.4'
}
```

It was published in com.facebook group

I requested sync to jcenter from bintray, that usually takes 2-3 days

Versioning added version suffixes to aar output files and circleCI jobs for android start failing as they expected just pytorch_android.aar pytorch_android_torchvision.aar, without any version

To avoid it - I changed circleCI android jobs to zip *.aar files and publish as single artifact with name artifacts.zip, I will add kostmo to check this part, if circleCI jobs finish ok - everything works :)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25351

Reviewed By: kostmo

Differential Revision: D17135886

Pulled By: IvanKobzarev

fbshipit-source-id: 64eebac670bbccaaafa1b04eeab15760dd5ecdf9

.gitignore

@@ -9,6 +9,7 @@
 ## PyTorch
 
 .coverage
+.gradle
 .hypothesis
 .mypy_cache
 */*.pyc