So, I need those who respond to start actually reading through my other posts on this question before posting. I think you're missing A LOT of updates about where I'm at with this and what I've covered. If that's too much to ask, I'll just resubmit a new question, because after a month I don't see any progress being made.

That very first post where I was getting the 403 error, was a result of the dataset not being uploaded successfully in the first place. NOT AS A RESULT OF AN ISSUE WITH THE ACCESS TOKEN.

403 is a generic Forbidden error. It doesn't mean anything more than that. It certainly is not a definite indicator that the issue is with an acess token. ANYONE RESPONDING NEEDS TO CEASE RELYING ON THAT ARGUMENT.

But your code is different from what I was trying, so I figured I give it a go.

First of all, here's what I was getting before I tried your solution:

If you right click on that image, you can open a larger version and see that I'm getting a "Failed to download the dataset from the public URL path".

When I tried out your code, here's what I got:

If you look at that, you'll see I'm getting an "Invalid access token" response with your code.

My first lightning app is still working just fine with what I set up, and I've had no problems with the other ways I've set up access tokens for my other calls through curl. I have so far not encountered anything to make me think any of this is an issue with the access token. But because I know you all have your doubts, here's more proof.

Here's a screenshot of my first lightning app's access token working just fine today.

Here's the code for creating the request to the metamind api:

    @AuraEnabled
    public static String getCallVisionUrl(string myURL) {
        // Get a new token
        JWT jwt = new JWT('RS256');
        jwt.cert = 'SelfSignedCert_18Jul2016_235259';
        jwt.iss = 'developer.force.com';
        jwt.sub = 'joseph.daily@careerbuilder.com';
        jwt.aud = 'https://api.metamind.io/v1/oauth2/token';
        jwt.exp = '3600';
        String access_token = JWTBearerFlow.getAccessToken('https://api.metamind.io/v1/oauth2/token', jwt);                
        String VISION_API = 'https://api.metamind.io/v1/vision';
        String PREDICT = VISION_API + '/predict';
        string contentType = HttpFormBuilder.GetContentType();
        //  Compose the form
        string form64 = '';
        
        form64 += HttpFormBuilder.WriteBoundary();
        form64 += HttpFormBuilder.WriteBodyParameter('modelId', EncodingUtil.urlEncode('GeneralImageClassifier', 'UTF-8'));
        form64 += HttpFormBuilder.WriteBoundary();
        form64 += HttpFormBuilder.WriteBodyParameter('sampleLocation', myURL);        
        form64 += HttpFormBuilder.WriteBoundary(HttpFormBuilder.EndingType.CrLf);
        
        blob formBlob = EncodingUtil.base64Decode(form64);
        string contentLength = string.valueOf(formBlob.size());
        //  Compose the http request
        HttpRequest httpRequest = new HttpRequest();
        
        httpRequest.setBodyAsBlob(formBlob);
        httpRequest.setHeader('Connection', 'keep-alive');
        httpRequest.setHeader('Content-Length', contentLength);
        httpRequest.setHeader('Content-Type', contentType);
        httpRequest.setMethod('POST');
        httpRequest.setTimeout(120000);
        httpRequest.setHeader('Authorization','Bearer ' + access_token);
        httpRequest.setEndpoint(PREDICT);
        system.debug('httpRequest: ' + httpRequest);
        system.debug('httpRequest header: ' + httpRequest.getHeader('path'));
        system.debug('httpRequest body: ' + httpRequest.getBody());
        
        Http http = new Http();
        HTTPResponse res = http.send(httpRequest);
        system.debug('res: ' + res);
        system.debug('res.getBody(): ' + res.getBody());
        return res.getBody();
    }    

My original authentication is still working for that request, so why would the same access token not work for other requests, especially from the same sandbox? 

Ignoring that glaring question no one has answered, let's move on to the other ways of generating and using an access token. Here's me generating a token using the pem file like you suggested. 

Here's me using the generated access token to upload a zip file:

Continued in next post due to image restrictions....