Repository: tobymao/sqlglot

Issue:
Teradata: TOP N is not supported
Thankyou for SQLGLOT. Fantastic library.

The below fails 
```python
import sqlglot

sqlglot.transpile("select top 10 column1 from table1.column1", identity=True, read="teradata")
```

***Error***
```
sqlglot.errors.ParseError: Invalid expression / Unexpected token. Line 1, Col: 12.
  select top 10 column1 from table1.column1
```

**Official Documentation**
[Teradata SQL reference](https://docs.teradata.com/r/2_MC9vCtAJRlKle2Rpb0mA/__1TX6kDYjUS0vA5Hi0Y0g)

I will be happy to contribute the fix but I am new here and would like some guidance on where to start. If my guess is right, I will have to make changes only to the `KEYWORDS` dict in `dialects/teradata.py` to add `"TOP": TokenType.TOP` but I am not entirely sure. I will watch and learn for the next time. 

Thanks again for this library. 


Discussion:

Patch:
diff --git a/sqlglot/dialects/teradata.py b/sqlglot/dialects/teradata.py
index d5e5dd81..d9a5417a 100644
--- a/sqlglot/dialects/teradata.py
+++ b/sqlglot/dialects/teradata.py
@@ -31,18 +31,19 @@ class Teradata(Dialect):
         # https://docs.teradata.com/r/Teradata-Database-SQL-Functions-Operators-Expressions-and-Predicates/March-2017/Comparison-Operators-and-Functions/Comparison-Operators/ANSI-Compliance
         KEYWORDS = {
             **tokens.Tokenizer.KEYWORDS,
+            "^=": TokenType.NEQ,
             "BYTEINT": TokenType.SMALLINT,
-            "SEL": TokenType.SELECT,
+            "GE": TokenType.GTE,
+            "GT": TokenType.GT,
             "INS": TokenType.INSERT,
-            "MOD": TokenType.MOD,
-            "LT": TokenType.LT,
             "LE": TokenType.LTE,
-            "GT": TokenType.GT,
-            "GE": TokenType.GTE,
-            "^=": TokenType.NEQ,
+            "LT": TokenType.LT,
+            "MOD": TokenType.MOD,
             "NE": TokenType.NEQ,
             "NOT=": TokenType.NEQ,
+            "SEL": TokenType.SELECT,
             "ST_GEOMETRY": TokenType.GEOMETRY,
+            "TOP": TokenType.TOP,
         }
 
         # Teradata does not support % as a modulo operator


Test patch:
diff --git a/tests/dialects/test_dialect.py b/tests/dialects/test_dialect.py
index 07ff4e47..76c04ae8 100644
--- a/tests/dialects/test_dialect.py
+++ b/tests/dialects/test_dialect.py
@@ -1300,6 +1300,7 @@ class TestDialect(Validator):
         self.validate_all(
             "SELECT x FROM y LIMIT 10",
             read={
+                "teradata": "SELECT TOP 10 x FROM y",
                 "tsql": "SELECT TOP 10 x FROM y",
             },
             write={