Update README and comments for standalone perplexity tool (#525)

Author: sw

README.md

@@ -248,7 +248,7 @@ cadaver, cauliflower, cabbage (vegetable), catalpa (tree) and Cailleach.
 
 ### Perplexity (Measuring model quality)
 
-You can pass `--perplexity` as a command line option to measure perplexity over the given prompt.  For more background,
+You can use the `perplexity` example to measure perplexity over the given prompt.  For more background,
 see https://huggingface.co/docs/transformers/perplexity.  However, in general, lower perplexity is better for LLMs.
 
 #### Latest measurements
@@ -271,10 +271,10 @@ Perplexity - model options
 #### How to run
 
 1. Download/extract: https://s3.amazonaws.com/research.metamind.io/wikitext/wikitext-2-raw-v1.zip?ref=salesforce-research
-2. Run `./main --perplexity -m models/7B/ggml-model-q4_0.bin -f wiki.test.raw`
+2. Run `./perplexity -m models/7B/ggml-model-q4_0.bin -f wiki.test.raw`
 3. Output:
 ```
-Calculating perplexity over 655 chunks
+perplexity : calculating perplexity over 655 chunks
 24.43 seconds per pass - ETA 4.45 hours
 [1]4.5970,[2]5.1807,[3]6.0382,...
 ```

examples/perplexity/perplexity.cpp

@@ -19,7 +19,7 @@ std::vector<double> softmax(const std::vector<float>& logits) {
 
 void perplexity(llama_context * ctx, const gpt_params & params) {
     // Download: https://s3.amazonaws.com/research.metamind.io/wikitext/wikitext-2-raw-v1.zip?ref=salesforce-research
-    // Run `./main --perplexity -m models/7B/ggml-model-q4_0.bin -f wiki.test.raw`
+    // Run `./perplexity -m models/7B/ggml-model-q4_0.bin -f wiki.test.raw`
     // Output: `perplexity: 13.5106 [114/114]`
     auto tokens = ::llama_tokenize(ctx, params.prompt, true);